Patent Publication Number: US-2021186528-A1

Title: System for Connecting a Connecting Device, in Particular a Distractor, to a Bone

Description:
The present invention relates to a system and method for connecting a connection device to a bone with a bone pin. The invention further relates to an external orthopaedic device, in particular a distractor, connecting different bones or bone parts. The invention further relates to a method for connecting an external distractor between at least two bone parts of a patient. 
     In the field of trauma surgery as well as in orthopaedics, external fixators are commonly used for treatment of complex fractures, stabilization of bony structures or joints, and in bone lengthening procedures. In external fixation, multiple bones or bone fragments are connected with a frame for providing additional structure to the bones, where fixation is achieved with bone pins, which can partially or fully penetrate the bone, providing one or more options per bone pin for attachment of an external device. More complex approaches comprise the use of ring fixators, or Ilizarov frames, which enable fixation of multiple bones with bone pins or tensioned wires, such that bones are reachable from nearly any direction preferably perpendicular to the longitudinal bone axis. For fixation of less bone parts, unilateral fixators, generally connected with multiple sets of two or three bone pins, may provide the required additional structure for a treatment. 
     Various frame designs provide, besides fixation, additional features like adjustability of the frame for reconfiguration of the fixated bone parts after fixation and without replacing the bone pins, dynamization between the entry sites for stimulation of bone growth, or joint motion in case of fixation of multiple bones forming a joint. Recent development introduced externally fixated devices in treatment strategies for joint diseases like osteoarthritis where affected joints can be bridged by a dedicated external fixator for application of joint distraction. 
     Osteoarthritis is a degenerative joint disorder, progressive over time, characterized by cartilage damage and loss, changes in peri-articular bone, synovial inflammation, and peri-articular muscle and ligament changes. These tissue changes are considered causative for clinical characteristics like pain, stiffness, and functional disabilities (Bijlsma J W J et al. Lancet 2011; 377: 2115-26). Knee osteoarthritis is the most prevalent form of osteoarthritis with an increasing socioeconomic and healthcare burden. Due to, amongst others, aging of the population and increase in obesity at a young age occurrence of knee osteoarthritis is estimated to increase significantly in the forthcoming years. 
     Few options are available for treatment of end-stage knee osteoarthritis and none have clearly been shown to halt or reverse tissue structure damage. Conservative treatment focuses on minimizing functional disability and pain relief with anti-inflammatory treatment when indicated (Bijlsma J W J et al. Lancet 2011; 377: 2115-26). 
     In case of uni-lateral tibio-femoral knee osteoarthritis, high tibial osteotomy or unilateral knee arthroplasty as well as new options like partial medial joint unloading by subcutaneous placed devices may provide a temporary solution. However, these techniques are rather invasive. Most importantly these techniques do not serve bi-lateral tibio-femoral knee osteoarthritis. 
     Eventually for all these cases a total knee prosthesis is indicated, and joint tissue is removed from the bony ends and replaced by artificial joint components. Unfortunately, these endo-prostheses have a limited life span due to wear and tear of the materials. The younger the patients (the more active they still are) the shorter the life span of a knee prosthesis. In case placed at an age below 65 years the chance for revision surgery is high. These revision surgeries are much more complex and expensive with less good outcome. 
     Therefore, there is a need for joint preserving treatments, to delay the first total knee prosthesis and with that prevent revision surgery. For specifically such cases, knee joint distraction has been introduced as a bi-lateral tibio-femoral joint preserving procedure. In case of failure of this treatment, the joint is uncompromised and any other treatment, including surgical intervention like hemi prosthesis, osteotomy or a total knee prosthesis remains optional. 
     Knee joint distraction has quite recently been introduced as a tibio-femoral joint preserving procedure and is more and more accepted as option to postpone placement of a first prosthesis below the age of 65 years. Both clinical improvement and structural tissue regeneration have been demonstrated with clinically relevant duration. (Mastbergen S C et al Nat Rev Rheumatol. 2013; 9: 277-90). This surgical treatment gradually separates the bony ends of a joint to a certain extent, increasing the joint space width with approximately five millimetres, for a limited period of 6-8 weeks. 
     Separation of the bony ends is achieved with an externally fixated device, connected to the bones forming the joint, with in general half bone pins drilled through soft tissues into both bone cortices. For a stable fixation bone pins are placed at multiple locations in each bone, in case of the knee at the medial and lateral side of femur and tibia. The external device is connected to these bone pins, as such forming a rigid structure enabling fixation of the whole joint and enabling distraction of the joint. Joint loads during application are primarily transduced via the external device and the bone pins, relieving the affected joint. In the distracted configuration, a confined axial displacement remains present when loaded and unloaded, still preventing further wear and tear of the damaged articulating surfaces with the resilience potentially stimulating the regenerating capability of the joint. 
     As primary functionality, external fixators provide additional stiffness to a bony structure. Stiffness is established by interconnecting bony parts via bone pins to a frame with specifications that suit the chosen treatment strategy. The connection between the external frame, in particular a connection device thereof, and the bone pins should be ensured in order to have advantage of other system features. Due to the variation in optimal position between patients and anatomical sites, establishment of the connection between the bone pins and the frame often requires multiple actions like tightening of multiple bolts from multiple directions, while stability of the connection is not always ensured. Reusability of external devices, and accompanying signs of use like burrs and wear from the cleaning and sterilization procedures, often worsen the reliability of such connections. Since placement of a bone pin requires at least one line of action for attachment to a bone, it is ensured that for every placed bone pin, a certain position and orientation provides sufficient reachability for performing other actions like fixation of the pin to the external frame. A solution which enables use of this ensured line of action provides a more intuitive and easy way of working for the surgeon. Moreover, the established connection can become more reliable when critical components receive inspection previous to every usage, or even are replaced. 
     A problem associated with known external orthopaedic devices including a system for connecting a connection device to the bone is therefore the heaviness and bulkiness of the existing solutions to frame the bones, because of the use of many components. Moreover, existing solutions require a time consuming surgery process to place the frame optimally to the human body, because of the use of (many) external tools for placement and assembling various system components to a functional structure. A further problem associated with known connection systems is that incorrect or non-optimal placement of bone pins may result in undesirable stress levels in the pin and in the bone structure. 
     Fixation of the device with half-pins into the bones of the treated joint requires anatomically optimal positioning at each of the bony ends of the joint of interest, such that bone pins do not intersect or interfere with critical tissue structures like nerves and blood vessels, but also not with tendons and muscle bundles and with that reducing the risk for complications or permanent damage. Furthermore, bone pins should not enter the joint capsule. Additionally important bone pins should avoid the area where later in life a prosthesis needs to be placed, so they should be placed at a certain distance from the joint space. Clearly there are for all joints a limited number of preferable anatomical locations where bone pins should be placed for optimal result with minimal damage. 
     As a result of the preferred anatomically available bone pin locations at each side of the joint and at the different sides of each bone, bone pins might need positional ‘misalignment’ with respect to mechanics of the external fixation frame, to obtain optimal anatomical positions of the bone pins. In case bone pins are positioned for fixation and distraction of the knee joint, angular misalignment around the mechanical axis of the joint results from the choice for optimal anatomical pin placement. 
     The interaction of the bone pins with soft tissues is therefore a further problem associated with known external fixation devices such as external distractors. The pin positions and orientation are dictated by the device, such that pins may intersect with fragile soft tissue. Moreover, axial displacement cannot be guaranteed to stay parallel with the longitudinal axis of the tibia, which is preferred. If axial displacement is not parallel to the longitudinal axis of the tibia, the interdistance of the frame and the human leg increases in the length direction of the femur, which is not favourable regarding joint configuration, and which is making this construction uncomfortable for the patient to wear. 
     It is a goal of the present invention, next to other goals, to provide a system for connecting a connection device to a bone wherein at least one of the above mentioned problems is solved or at least alleviated. 
     This goal, amongst other goals, is met by a system according to appended claim  1 . More specifically, this goal, amongst other goals, is met by a system for connecting a connection device to a bone with a bone pin, wherein the connection device is provided with an opening for receiving the bone pin, wherein the system comprises:
         said connection device;   a guiding tube arranged to guide the bone pin from the connection device to the bone for connecting the bone pin to the bone, wherein the inner diameter of the guiding tube corresponds to the outer diameter of the bone pin and wherein the guiding tube can be slidably received in the opening, the guiding tube being slidable with respect to the connection device along an axis parallel to a longitudinal axis of the opening;   a locking device arranged to be received in the opening of the connection device and arranged to engage the bone pin for locking the bone pin with respect to the connection device,
 
wherein the guiding tube is movable with respect to the connection device and bone pin such that the guiding tube is removable from the combination of the bone pin and the connection device and wherein the locking device is arranged to lock the bone pin after removal of the guiding tube.
       

     The system for connecting a bone pin to a connection device according to the invention allows the connection device, for instance in the form of a connection block of an external orthopaedic device, to be used as a drilling guide such that the trajectory of the bone pin is determined by the configuration of the connection device, in particular the orientation of the opening thereof. After proper guidance of the bone pin using the opening of the connection device, the bone pin can be efficiently locked to the connection device using a locking device which is shaped to be received in the opening of the connecting device and which is arranged to lock the bone pin with respect to the connection device. A simple and efficient connection system that is intuitive to use is herewith provided. 
     Inserting the bone pins by drilling needs protection of the soft tissues during drilling by a guiding tube, sometimes also referred to as a cannula or sleeve, around the bone pin protecting soft tissue to be harmed by the rotating bone pin during drilling. To enable drilling of a bone pin through the connection device using the guiding tube and subsequent fixation of the bone pin after removal of the guiding tube to the connection device, a locking device is provided. In a preferred embodiment, rotation of the guiding tube is blocked during drilling. 
     The invention also relates to a method for connecting a connection device to a bone with a system according to invention, wherein the method comprises the steps of:
         providing a bone pin in said guiding tube in the opening of the connection device;   guiding the guiding tube to the bone by sliding said guiding tube in the opening of the connection device with respect to said connection device, such that the guiding tube abuts the bone;   inserting the bone pin into the bone;   removing the guiding tube from the combination of the bone pin and the connection device after insertion of the bone pin; and   locking the bone pin with respect to the connection device with said locking device after removal of the guiding tube.       

     The opening is arranged to slidably receive a guiding tube. This allows adjusting the relative position of the guiding tube to be adjusted with respect to the connection device. It is in particular preferred that the depth of the guiding tube in the patient can be adjusted, such that the guiding tube is adjusted such that the distal end of the guiding tube abuts the bone. This prevents damage to the tissue surrounding the bone upon drilling of the bone pin. 
     The connection system according to the invention is in particular suitable to connect a bone pin in accordance with a pre-planned trajectory. This trajectory is defined by the orientation of the opening of the connection device. This solves or at least alleviates the problem of incorrect placement of the bone pins, resulting in undesirable stress levels in the pin and in the bone structure, as well as on the device. The guiding tube is therefore preferably slidable in the opening along only the longitudinal axis of the opening. Any other relative movement is preferably prevented. The step of guiding the guiding tube therefore preferably comprises keeping the connection device substantially stationary with respect to the patient. 
     According to a preferred embodiment, the locking device has an annular body having an outer diameter corresponding to the diameter of the opening of the connection device and an inner diameter suitable for engaging the bone pin. The outer surface having the outer diameter is then arranged to efficiently connect to the connection device due to the matching diameters. As an alternative, or in addition thereto, it may however also be possible that the locking device and the connection device interconnect at another location than the outer surface of the locking device. 
     The inner surface of the locking device is then arranged to be able to lock the bone pin by engagement. This prevents movement between the locking device and the bone pin, such that also relative moment of the bone pin with respect to the connection device is prevented. 
     In order to achieve an efficient locking of the bone pin, the locking device according to a preferred embodiment comprises an engaging surface for engaging the bone pin, wherein the engaging surface is movable towards and from said bone pin between an unlocked position, wherein the bone pin is movable with respect to the engaging surface, and a locking position, wherein the engaging surface engages the bone pin for locking said bone pin with respect to the connection device by clamping. The step of locking the bone pin in the method according to the invention therefore preferably comprises engaging the bone pin with an engaging surface of the locking device for clamping said bone pin. The engagement surface is preferably moveable radially inwardly towards the longitudinal axis of the bone pin. 
     It may be possible that the engagement surface is only movable from the unlocked position to the locked position, such that the bone pin can no longer be loosened after locking with the locking device, preferably with (but not restricted to) plastically deforming either of the components, preferably the locking device. It is however preferred if the engaging surface is also movable from the locked to the unlocked position, such that the bone pins can be loosened if needed. 
     The engaging surface may for instance be formed on movable or deformable parts of the locking device, such as tongues, which are movable towards and preferably also from the bone pin for locking the bone pin. 
     Preferably, the locking device has an inner diameter substantially corresponding to the diameter of the bone pin. In this embodiment, the locking device can only be placed in the opening after the removal of the guiding tube, as the guiding tube would not fit in the locking device. Providing a locking device having an inner diameter corresponding to the outer diameter of the bone pin allows a good locking action, as for instance the engagement surface only needs to be displaced over a little distance to obtain a good clamping action. 
     As an alternative, the locking device has an inner diameter substantially corresponding to the outer diameter of the guiding tube in the unlocked position. This allows the locking device to be already in place in the opening when the guiding tube is guided in the opening. The locking device may then have a guiding function for guiding the guiding tube. When the guiding tube is removed after insertion of the pin in the bone, the engagement surface is moved towards the bone, thereby bridging the wall thickness of the then removed guiding tube, to clamp the bone pin. 
     In order to be able to efficiently move the engaging surface to the locked position, the opening of the connection device preferably has a tapering diameter, seen along the longitudinal axis of the opening, for moving the engaging surface between the positions, upon longitudinal movement of the locking device with respect to the connection device. The step of locking the bone pin in the method according to the invention then preferably comprises moving the locking device along the longitudinal axis of the opening of the connection device for urging the engaging surface onto the bone pin. The tapering diameter, or at least varying diameter, of the opening then serves as a guiding surface for guiding the engagement surface towards the bone pin for engagement. To further improve the movement of the engaging surface, the locking device preferably comprises a correspondingly shaped tapering outer diameter. 
     As an alternative, it is also possible that only the locking device has a varying outer diameter, for instance a wedge shape, which urges the engaging surface onto the bone pin upon movement of the locking device into the opening. 
     According to a further preferred embodiment, the locking device comprises a deformable element arranged to deform in the locked position for engaging the bone pin. The deformable element may for instance be manufactured from a resilient material such as rubber or flexible plastic. 
     The locking device may be formed as a single unit. This reduces the number of parts. It is however also possible that the locking device comprises an engaging element provided with at least one engaging surface and a separate driving element arranged to move the engaging element along the longitudinal axis of the opening for moving the engaging surface from the unlocked to the locked position. It is for instance possible that the driving element is removed from the opening after locking the bone pin with the engaging element. This prevents accidental loosening of the bone pin. 
     It is however preferred when the engagement element is arranged to be moved to the locked position upon movement in the opening along the longitudinal axis as explained above, that the engaging element comprises at least one second engaging surface at a distance from the first engaging surface, seen along the longitudinal axis of the opening. This allows clamping at two locations of the bone pin, thereby improving the locking action. The driving element is then preferably provided with an opening for receiving the engaging element and wherein the opening of the driving element has a tapering diameter, seen along the longitudinal axis of the opening, for moving the second engaging surface between the unlocked and locked position upon longitudinal movement of the driving element with respect to the engaging element. Also the engaging element and the driving element are then provided with a cooperating guiding surfaces for urging the engaging surface towards the bone pin. As said, this improves the locking action. 
     According to a further preferred embodiment, the opening comprises, seen along the longitudinal axis of the opening, a first section having a first diameter and a second section having a second diameter corresponding to the outer diameter of the guiding tube, wherein the first diameter is larger than the second diameter. The change in diameter can be used to move the engaging surface towards the bone pin as mentioned above. The broader section moreover allows receipt of the locking device. To efficiently move the engaging surface as mentioned above, it is further preferred if the opening comprises a tapering section extending between the first and the second sections in which the diameter tapers from the first diameter towards the second diameter. It may be possible that the second section has a limited length, such that the tapering section extends until the end, wherein diameter at the end corresponds to the second diameter. 
     According to a further preferred embodiment, the opening of the connection device and the locking device are provided with cooperating connecting means for interconnecting the locking device and the connection device. This improves the locking action. Several types of connecting means may be used. It is for instance possible to use a bayonet connection between the locking device and the connection device having a resilient lip engaging in a correspondingly shaped recess or groove. For instance in the case a deformable part is used, it is preferred if the opening is provided with a recess, preferably a groove, for receiving the deformable element for locking the connection device and the locking device. 
     Efficient locking, while allowing easy manipulation, is however obtained if the locking device and the connection device are provided with cooperating threading. 
     In case a driving element is used, it is preferred if the driving element and the opening of the connection device are provided with cooperating connecting means. This then locks the driving element with respect to the connection device, while the locking element and the driving element are further preferably also provided with cooperating connecting means. 
     In order to provide sufficient room for the connecting means, it is preferred if the connecting means are provided in the first, broader section of the opening. 
     As mentioned above, it is possible that the locking device is arranged to receive and guide the guiding tube. The locking device thereto has a suitable inner diameter. It is however also possible to use a separate guiding element, such that the locking device is dedicated to lock the bone pin. The inner diameter of the locking device may then correspond to the outer diameter of the bone pin as mentioned above. Therefore, a further preferred embodiment of the system further comprises a guiding element for guiding the guiding tube, wherein the guiding element is arranged to be received in the opening of the connection device, wherein the guiding element has an outer diameter corresponding to the diameter of the opening and wherein the guiding element has an inner diameter corresponding to the outer diameter of the guiding tube, wherein the guiding element and the guiding tube are removable from the combination of the bone pin and the connection device. The method according to the invention then preferably further comprises the step of inserting a guiding element in the opening of the connection device and arranging the guiding tube in said guiding element. 
     After proper guidance of the guiding tube, which in turns guides the bone pin to the bone, the guiding tube and the guiding element are removed. Preferably, the guiding element is removed prior to locking the bone pin. Also the guiding element preferably has an annular body. The outer diameter thereof preferably corresponds to the diameter of the opening, more preferably the annular body has an outer diameter corresponding to the diameter of the first section of the opening. The guiding element is then placed in the first section of the opening and serves to bridge the distance between the inner wall of the opening and the guiding tube. 
     In order to at least temporary fix the guiding element to the connection device to allow a stable guidance of the guiding tube, the guiding element is preferably provided with corresponding connecting means for interconnecting the guiding element and the connection device. The same connecting means as used for connecting the locking device in the opening are preferably used. 
     According to a further preferred embodiment wherein a separate driving element is used, said driving element is formed as the guiding element. This reduces the number of parts. 
     The system according to the invention is in particular useful for connecting a connection device to a bone with two parallel bone pins. A further preferred embodiment of the system according to invention therefore further comprises a second locking device, wherein the connection device comprises a second opening for receiving a second bone pin, wherein the longitudinal axes of the first and the second openings are substantially parallel and wherein the second locking device is arranged to lock the second bone pin after removal of a guiding tube from the second opening. The openings hereby define at least one of the trajectories of the bone pins to be inserted into the bone. 
     The invention thus further relates to a method for connecting a connection device to the bone with two parallel bone pins, comprising the step of connecting a connection device to the bone with a first bone pin and connecting the connection device to the bone with a second bone pin according to the method according to the invention. Using two parallel bone pins allows adjustment of distance of the connection device with respect to the patient, in particular when the locking device also allows loosening of the bone pins after locking said bone pin. 
     At least the second bone pin is connected in the above described manner, wherein the opening is used as drilling guide. After placement of a first bone pin at the most appropriate preferred anatomical position, the locations of additional bone pins is (to a certain extend) determined as they are all to be connected to the same external device. To facilitate guiding of the subsequent bone pins for fixation to an external device to the optimal anatomical bone pin positioning there is an advantage to use the for each application specifically designed connection device as bone pin positioning guide or drilling guide. In this way the optimal anatomical position of the bone pin is directly related to an optimal connection to the device and with that the mechanical positioning of the device. Moreover, the optimal anatomical positions can be found intuitively by the guidance of the connecting device. 
     The first pin may also be fixed using the system and method according to invention. It is however also possible that the step of connecting a connection device to the bone with a first bone pin comprises:
         inserting the bone pin into the bone using a guiding tube;   removing said guiding tube from the inserted bone pin;   arranging the connection device onto said inserted bone pin;   locking the bone pin with respect to the connection device with a locking device,
 
wherein the method further comprises subsequently connecting the second bone pin according to the method according to the invention. As the position of the connection device with respect to the patient is then fixed, it is an advantage that the slidable guiding tube is adjustable in depth to allow efficient protection of the surrounding tissue upon insertion of the second bone pin.
       

     The invention further relates to an external orthopaedic device arranged to be connected between a first bone part and a second bone part, wherein the orthopaedic device comprises a first connection device according to the invention and a second connection device, preferably also according to the invention. This orthopaedic device can be connected efficiently to the patient as discussed above. For each application this connecting device will be specifically shaped to guide the bone pins of the second connecting device intuitively to the preferred anatomical bone pin position and to provide optimal positioning of the second connecting device regarding mechanical characteristics of the entire configuration. 
     The invention further relates to an external orthopaedic device in the form of an external distractor arranged to gradually enlarge the distance between a first bone part and a second bone part, wherein the distractor comprises a first connection device, preferably as defined above, and a second connection device, preferably also as defined above, wherein the first connection device is arranged to be connected to the first bone part with at least one first bone pin and wherein the second connection device is arranged to be connected to the second bone part with at least one second bone pin, wherein the first and second connection devices are interconnected via at least a distraction device, the distraction device having an adjustable length for adjusting the distance between the first and the second connection devices along an adjustment axis. 
     The distractor device preferable comprises a first part and a second part which are movable with respect to each other along the adjustment axis. The parts may for instance be provided with cooperating threading to allow the adjustment of the length. The distractor device is preferably arranged to adjust the distance between the bone pins while maintaining the structural integrity of the distractor. The distance can thus be adjusted while transferring loads between the first and second bone parts. 
     The invention therefore also relates to a method for connecting an external distractor between at least two bone parts of a patient, comprising the steps of:
         providing an external distractor according to the invention;   connecting the first connection device to the first bone part with at least one bone pin; and   connecting the second connection device to the second bone part with at least one bone pin.       

     In particular when using the connection system according to the invention, the distractor can efficiently be connected to the patient. It is however noted that the invention is not necessarily limited to a distractor having connection devices according to invention. Addition of other connection devices, such as connection blocks which are known as such, may also be used. 
     In order to prevent further wear and tear of the damaged articulating surfaces and to potentially stimulate the regenerating capability of the joint, it is preferred if the distractor, in particular the distractor device thereof, comprises resilient means which are arranged for allowing resilient movement along the adjustment axis between the connection devices. The resilient means, for instance in the form of a spring or similar device, then allows small displacements along the adjustment axis. 
     Although it is possible that the distractor device directly couples the two connection devices, it is preferred if the external distractor further comprises an interconnecting system extending between the first and second connection devices, wherein the interconnecting system is movable between an unlocked position, wherein the connection devices are movable with respect to each other, and a locked position, wherein the mutual positions of the connection devices are fixed. The interconnecting system allows the connection devices to be moved with respect to each other which is in particular useful when connecting the distractor to the patient. 
     The interconnecting system may for instance comprises at least one lockable hinge, preferably two at either side connected to the connection devices. It is preferred that the interconnecting system is connected to the first and second connection devices with two respective lockable ball joints. This allows the connection devices to be efficiently moved with respect to each other. 
     A preferred method according the invention therefore preferably further comprises the steps of:
         providing the interconnecting system in the unlocked position;   aligning at least one of the connection devices along a predetermined axis with respect to a bone part; and   moving the interconnecting system to the locked position for fixing the mutual positions of the connection devices after alignment.       

     Preferably, the interconnecting system is moved to the locked position after fixation of the bone pins. 
     According to a further preferred embodiment, the distraction device is arranged in one of the connection devices, preferably integrally, wherein the interconnecting system extends between the distraction device and the other connection device. More preferably, the ball joint is formed as a part of the distractor device. The distraction device is hereby coupled between a connection device, and the interconnecting device. Although the distraction device may be formed integrally, it is also possible that the distraction device is insofar arranged in one of the connection devices that the minimal distance between the opening in the connection device and the adjustment axis of the distraction device is movable and can be fixed. The location of the opening in the connection device may for instance be adjustable along the adjustment axis of the distraction device to allow adjusting the length of the distractor upon connecting the distractor to the patient. 
     To allow efficient distraction, in particular of a joint, it is important that the distraction device is correctly aligned with the bone parts. It is in particular preferred if the adjustment axis is aligned with at least one of the longitudinal axes of the bone parts. The step of aligning therefore preferably comprises aligning the longitudinal axis of a bone to be substantially parallel to the adjustment axis of the distraction device. This alignment is more easy to perform if the distraction device is part of one of the connection devices as mentioned above, although this is not strictly necessary. 
     In order to allow connection of the connection device to a bone without damaging fragile soft tissue (at the anatomically preferred locations), while still allowing axial displacement parallel with the longitudinal axis of the bone (for optimal mechanical performance of the configuration), for instance the tibia, according to a further preferred embodiment the connection device is arranged to receive the bone pin at a certain distance from the adjustment axis. 
     In order to transduce loads over the joint via the distraction device, correction in alignment is required to ensure optimal unloading of the joint. A certain off-set of the bone pin connection to the axis of the device as well as preferably a certain curvature in the device as will be explained below to obtain optimal anatomical bone pin locations relative to optimal mechanical positioning of the external device will be needed. 
     According to a further preferred embodiment, at least one of the connection devices is arranged to be connected to the bone part with two substantially parallel bone pins. This improves the stability of the system, while the distance of the connection device and the patient is still adjustable as explained above. 
     At least one of the steps of connecting a connection device to the respective bone part according to the invention then comprises connecting the connection device to two substantially parallel bone pins. Preferably both connection devices are connected to the bone with two parallel bone pins. 
     The connection devices thereto preferably comprise two openings for receiving two bone pins. The step of aligning then preferably comprises aligning the connection device such that the openings in the connection device are arranged along an axis substantially parallel to the longitudinal axis of the respective bone part. This allows efficient alignment, wherein the openings in the connection devices are used as a reference for alignment. 
     It is then preferred if a connection device is provided with two openings for receiving the two bone pins, wherein the openings in the connection device are arranged along an axis at a distance from the pivot point of the ball joint. This creates an offset between the pivot point of the ball joint and the openings through which the bone pins are to extend. This offset allows an improved connection to the bone, at the preferred anatomical positions of the bone pins, preventing potential damage to sensible surrounding tissues. This offset may be specific for specific devices for specific joint bones. 
     For the connection device provided with the distraction device, it is preferred that when the connection device is provided with two openings for receiving the two bone pins, the openings in the connection device are arranged along an axis substantially parallel to the adjustment axis and such that the two bone pins extend substantially perpendicularly to the adjustment axis. The openings of the connection device can then be used to correctly align the adjustment axis with a predetermined axis of the bone, preferably the longitudinal axis of the bone. It is again preferred if an offset is created between the adjustment axis and an axis onto which the openings in the connection device extend. The openings in the connection device are therefore preferably arranged along an axis at a distance from the adjustment axis. 
     In particular for distracting a knee joint, it is preferred if one connection device is arranged to receive its bone pins at a distance in a first direction from the adjustment axis or the pivot point, wherein the second connection device is arranged to receive its bone pins at a distance in a second direction from the adjustment axis or the pivot point, the second direction being substantially opposite to the first direction. This protects sensible tissue when inserting the bone pins in accordance with the openings provided at an offset as explained above. Moreover, it provides intuitive positioning of the bone pins at the preferred anatomical locations, guided by the openings in the connection device. 
     In order to further adapt to the curvature of the patient, it is preferred if the connection device comprises a patient facing surface and an outer surface, wherein an opening extends between the patient facing surface and the outer surface, wherein at least the patient facing surface of the connection device has a curved cross-section, seen in a plane perpendicular to the axis trough the openings. The patient facing surface is then preferably curved in accordance with the curvature of the patient. This provides further intuitive positioning of the bone pins at the preferred anatomical locations, guided by the openings in the connection device. 
     The distractor is particularly suitable to be used to distract the knee joint. The invention thus also relates to a method for connecting an external distractor between the tibia and the femur, wherein one of the connection devices is connected to the tibia and the other connection device is connected to the femur. 
     When setting joint distraction to the distraction device, only an increasing joint space width is aimed for, while further joint configuration remains intact. To do so, the distraction direction needs to be orthogonal to both joint surfaces. In case of the knee this means perpendicular to the tibial surface and in the axial direction of the tibia. Specifically to the knee joint, the femoral joint side has a convex, spherical like, surface, while the tibial joint side has a more flattened concave corresponding surface. A direction of distraction orthogonal to both joint sides can be achieved, independent from flexion angle of the joint, by choosing the direction orthogonal to the least spherical surface, or orthogonal to the most flattened surface. 
     As the connection of the bone pins in the femur is more critical in terms of surrounding tissue which may be damaged, it is preferred if the femur is connected to its connection device prior to connecting the tibia to its connection device. 
     It is hereby possible that the first pin is inserted into the femur using a guiding tube, to subsequently connect the connection device according to the invention. A second bone pin can then be inserted into the bone using the connection device as drill guide after proper alignment of the connection device, in particular along the longitudinal axis of the bone. 
     According to a further preferred embodiment, the step of aligning comprises aligning the adjustment axis of the distraction device in the connection device to be substantially parallel to the longitudinal axis of the tibia in the coronal plane and preferably the sagittal plane substantially perpendicular to the tibial plateau. This provides an efficient distraction of the knee joint. 
     It is hereby noted that it is in particular important that the longitudinal axis of the tibia is aligned with the adjustment axis. It may for instance be possible that the patient is not capable of completely extending the knee, such that the alignment axis is not properly aligned with the longitudinal axis of the femur, such that the connection device on the femur is not aligned with the longitudinal axis of the tibia. The interconnection system then still allows proper alignment of the connection device on the tibia, such that the adjustment axis is parallel to the longitudinal axis of the tibia as explained above. 
     It is noted that the term aligned as used herein is to be understood that the axes to be aligned are substantially parallel, that is within ±10 degrees with respect to each other. 
     It is hereby preferred if each of the connection devices is provided with two openings for receiving the two bone pins, wherein the connection devices are aligned such that the openings in the connection device are arranged along axes substantially parallel to the longitudinal axes of the respective bones and such that the two bone pins extend substantially perpendicularly to the longitudinal axes of the respective bones. In order to prevent damage to tissue as already mentioned above, it is then preferred if the step of aligning the connection devices further comprises aligning the adjustment axis and/or the pivot points of the ball joints substantially on the coronal plane, and:
         orienting the openings of the connection devices to be connected to the femur in a posterior position; and/or   orienting the openings of the connection devices to be connected to the tibia in anterior position.       

     The distractor as explained may be used as an unilateral distractor. It is however preferred to use the system as a bilateral system for application of knee joint distraction. The invention thus also relates to a method for connecting an external distractor system between at least two bone parts of a patient, comprising the steps of connecting a first external distractor according to the invention at a first side of the bone parts and connecting a second external distractor according to invention at a second side, opposite the first side of the bone parts. It is hereby preferred if the first and second external distractors are arranged substantially on the similar coronal plane on either side of the knee. 
    
    
     
       The present invention is further illustrated by the following Figures, which show a preferred embodiment of the device and method according to the invention, and are not intended to limit the scope of the invention in any way, wherein: 
         FIGS. 1A-1D  schematically show a system and method for connecting a connection device to a bone; 
         FIGS. 2A-2D  schematically show a variant of the system of  FIGS. 1A-1D ; 
         FIGS. 3A and 3B  schematically show a variant of fixing a bone pin to a connection device; 
         FIG. 4  shows a variant of a connection device having two openings; 
         FIG. 5  shows a distractor according to the invention; 
         FIGS. 6A-6C  shows a first connection device of the distractor of  FIG. 5  in perspective, top view and side view respectively; 
         FIGS. 7A-7D  shows a second connection device with distraction mechanism in perspective, top view in two positions and in side view respectively; 
         FIG. 8  shows the positional relationship of the bone pins of the two connection devices in side view; 
         FIGS. 9A-9B and 10A-10B  show a joint knee and the femur and tibia in cross section indicating the bone pin positions; and 
         FIGS. 11A-11C  show a bilateral distraction system according to the invention. 
     
    
    
     In  FIGS. 1A-1D  the steps for connecting a connection device  1  in the form of a connection block to a bone  100  is shown. The connection block  1  is thereto provided with an opening  11  which extends between a first surface  12 , which faces the patient  101 , and a second surface  13  which faces away from the patient  101 . The opening  11  has a section  11   a  towards the second surface  13  which is provided with threading  14 . Towards the patient facing surface  12 , the opening  11  is provided with a tapering section  11   b  wherein the diameter of the opening  11  becomes smaller in the direction of the patient  101 , seen along a longitudinal axis A of the opening  11 . 
     Inserted into the opening is a locking device  2  which has a substantially annular shape and is shaped to fit inside the opening  11 . The locking device  2  has a flange  21  at one side and deformable tongues  22  at the other side and a body  23  provided there between. The body  23  is provided with threading  24  for cooperation with the threading  14  of the opening. The outer diameter of the body  23  of the locking device  2  thereby corresponds to the diameter of the first section  11   a  of the opening  11 . The length of the body  23  provided with the threading  24  preferably corresponds to the length of the first second  11   a  of the opening  11 , seen along the longitudinal axis A. 
     The threading  14 ,  24  fixes the relative position of the locking device  2  and the connection device  1 , which position can be adjusted by rotating the locking device  2 . In the position as show in  FIG. 1A , the lower surface of the flange  21  extends at a distance from the surface  13 , such that further movement of the locking device  2  in a direction indicated with I in  FIG. 1A  is possible, as will be explained in greater detail below. In the situation as shown in  FIG. 1A , the tongues  22  of the locking device  2  only partially extend in the tapering section  11   b  of the opening  11 . 
     The inner diameter of the locking device  2 , including the diameter at the location of the tongues  22 , corresponds to the outer diameter d 1  of a guiding cannula  3 . This limits relative movement of the locking device  2 , which also serves as a guiding device for guiding the cannula  3 , in a direction along the longitudinal axis A. The guiding cannula  3  is arranged to guide a bone pin  4  from the connection device  1  to the bone  100 , see  FIG. 1B . In inserted situation as shown in  FIG. 1B , the cannula  3  is thereto slidable along the longitudinal axis A with respect to the locking device  2  and thereby with respect to the connection device  2 . A flange  32  is provided to allow efficient adjustment of the depth of the cannula  3 . This allows efficiently guiding a bone pin  4  to the bone  100 , irrespective of the distance between the connection device  1  and the bone  100 . When the cannula  3  is advanced sufficiently far such that a distal end  31  abuts the bone  100 , the bone screw  4  can be inserted into the cannula  3 , schematically indicated with the arrow in  FIG. 1B . The inner diameter of the cannula  3  thereto corresponds to the outer diameter of the bone pin  4  such that relative movement of the bone pin  4  in the cannula  3  is again restricted to movement along the longitudinal axis A. The movement of the bone pin  4  with respect to the connection device  1  is therefore also fixed. 
     In a next step, see  FIG. 1C , the bone pin  4  can be fixed to the bone  100 . In this example, self-tapping bone screws  4  are used, such that the bone pins  4  are inserted into the bone  100  by rotating the bone pin  4  inside the cannula  3 . The cannula  3  thereby prevents damage to the surrounding tissue of the patient  3 . Other bone pins  4  can however be used. 
     After fixation of the bone pin  4  to the bone  100 , the cannula  3  can be withdrawn from the combination of the bone pin  4 , the locking device  2  and the connection device  1 . This movement is indicated with the arrow in  FIG. 1C . As the inner diameter of the locking device  2  corresponds to the outer diameter of the cannula  3 , which is larger than the outer diameter of the bone pin  4 , the bone pin  4  is held in the locking device  2  with play. Therefore, in order to lock the bone pin  4  with respect to the locking device  2  and thereby with respect to the connection device  1 , the locking device  2  is rotated, thereby moving the locking device  2  along the longitudinal axis A, see arrow in  FIG. 1D , in the opening  11 . The outer surfaces of the tongues  22  will thereby engage the inner surface of the tapering section  11   b , urging the tongues  22  radially inwardly, i.e. towards to the longitudinal axis and thereby towards the bone pin  4 . The inner surface of the deformable tongues  22  thereby act as engaging surfaces which are arranged to engage the bone pin  4 , thereby locking the bone pin  4  with respect the locking device  2  and thereby the connection device  1 . In the locked position as shown in  FIG. 1D , the lower surface of the flange  21  of the locking device  2  abuts the upper surface  13  of the connection device  1 , which is in this example provided with a correspondingly shaped recess to receive the flange  21  in a countersunk manner. 
     In this situation, the bone pin  4  is firmly locked with respect to the connection device  1 . It will be appreciated, as will be explained in greater detail below, that the bone pin  4  can simply be unlocked by rotating the locking device  2 , thereby loosening the tongues  22  such that the locking device  2  is again movable with respect to the bone pin  4 . 
     In the example shown in  FIGS. 1A-1D , the locking device  2  serves to lock the bone pin  4  and to guide the cannula  3 . It is however also possible to use a separate guiding device to guide the cannula  3 . Moreover, in the above example, the relative position of the locking device  2  and the connection device  1  is fixed using threading  14 ,  24 . Other means can however be used as will also be shown with reference to  FIGS. 2A-2D . 
     The connection device  1  as shown in  FIGS. 2A-2D  is similar to the connection device  1  as shown in  FIGS. 1A-1D  and is again provided with an opening  11 . The diameter of this opening  11  is again larger than the outer diameter d 1  of the cannula  3 , such that a guiding device  5  is used to limit the relative movement between the cannula  3  and the connection device  1  along the longitudinal axis A, see  FIG. 2B . The guiding device  5  again has a annular shape and has an inner diameter corresponding to the outer diameter d 1  of the cannula  3 . This allows efficient adjustment of the depth of the cannula  3  with respect to the connection device  1  as indicated with double arrow in  FIG. 1B . Note that this is different from the locking device  2  as used in  FIGS. 1A-1D . 
     Instead of threading, the guiding device  5  is provided with a deformable part  51  which locks into the opening  11  upon inserting the part  51 . The deformable part  51  thereby exerts a clamping action of the inner surface of the opening  11 , thereby retaining the guiding device  5  in the opening  11  by friction. The guiding device  5  is further provided with a flange  52  for easy manipulation of the guiding device  5 . 
     After insertion of the bone pin  4 , see  FIG. 2C , the combination of the guiding device  5  and the cannula  3  can be removed from the combination of the bone pin  4  and the connection device  1 , see arrow  FIG. 2D . As the bone pin  4  is firmly attached to the bone, the combination can be withdrawn by pulling sufficiently hard to overcome the friction of the deformable part  51  of the guiding device  5 . The cannula  3  and the guiding device  5  can hereby be removed in unison. 
     Although a locking device similar to the locking device as shown in  FIGS. 1A-1D  can be used to lock the bone pin  4  with respect to the connection device  1 , for instance by providing corresponding threading, it is also possible to use a locking mechanism formed of two separate parts, as is shown in  FIGS. 3A and 3B . 
     The locking mechanism comprises a driving element  6  and a locking element  7 , wherein the annularly shaped locking element  7  is provided with sets of tongues  71 ,  72  at either side. Both sets of tongues  71 ,  72  are arranged to move radially inwardly upon deformation, thereby locking the bone pin  4 . The first set of tongues  71  at the lower side of the locking element  7  are moved radially inwardly upon moving, or driving, the locking element  7  in a direction indicated with the arrow in  FIG. 1A . The reduction of the diameter indicated with  19  in the opening  11  will thereby urge the tongues  71  towards the outer surface of the bone pin  4 . A groove  11   c  is provided near end of the opening  11  towards the patient, wherein ribs  71   a  of the tongues  71  can snap for locking the relative positions of the locking element  7  with respect to the connection device  1 . In this locked position, the tongues  71  exert a firm clamping action on the bone pin  4  at a location along the longitudinal axis A indicated with the arrow C in  FIG. 3B , thereby preventing movement of the bone pin  4 . 
     To further improve the locking action, also to the tongues  72  provided on the upper side of the locking element  7  will be urged radially inwardly upon movement of the driving element  6  towards the locking element  7 . More specifically, the annularly shaped driving element  6  is provided with an opening  61  of which the lower section  64  has a tapering diameter, wherein the diameter increases towards the lower side. The tapering section  64  forms a guiding surface for urging the tongues  72  inwardly, i.e. towards the bone pin  4 , when the driving element  6  is moved towards the locking element  7 . 
     Also the tongues  72  can be provided with ribs  72   a  which can be received in a correspondingly shaped groove  63  at the end of the guiding surface. This connects the driving element  6  to the locking element  7  and thereby to the connecting device  1 . In the situation as show in  FIG. 3B , the tongues  72  exert a clamping action on the bone pin  4  at a location along the longitudinal axis A indicated with B, at a distance from location C. This improves the locking action of the locking mechanism. 
     The locking system may but not necessarily make use of ribs ( 71   a / 11   c  and  72   a / 63 ) and corresponding grooves at either side or at both sides, to fix the  7  into the connecting device  1  or the driving element  6  or both. 
     The connection system as described above is particularly suitable to connect a connection device to a bone with two parallel bone pins. This is explained with reference to  FIG. 4 . Although in this figure a connection system is shown which corresponds to the system as shown in  FIGS. 1A-1D , it will be appreciated that the same applies to the system as shown in  FIGS. 2A-2D and 3A-3B , or combinations thereof. 
     In  FIG. 4 , a connection device  1  is shown provided with two parallel openings  11 , into which bone pins  4   a  and  4   b  are inserted. In this example, bone pin  4   a  was inserted first by using a separate cannula to drill the bone pin  4   a  into the bone  100 . After fixation of this bone pin  4   a  the connection device  1  was advanced over the proximal end  42  of the pin  4   a  and the pin  4   a  was locked with respect to the connection device  1  using a locking device  2   a . In order to ensure that a second pin  4   b  was inserted parallel to the first bone pin  4   a , the connection method similar to the methods shown in  FIGS. 1A-1D and 2A-2D  was used to insert and connect the bone pin  4   b . A cannula is thereby used to guide the bone pin  4   b  to the bone  100 , the surface of which differs in terms of height from the surface of the first bone pin  4   a  as shown in  FIG. 4 . 
     In this example however, a separate guiding device was used (similar to the method of  FIGS. 2A-2D ) to guide the cannula, although a locking device in one piece was used (similar to the locking device of  FIGS. 1A-1D ). Moreover, the interconnection of the locking devices  2   a  and  2   b  (and the guiding devices which are not shown) with the connection device  1  is obtained with corresponding threading, again similar to the embodiment of  FIGS. 1A-1D . The situation of the bone pin  4   b  as shown in  FIG. 4  thus corresponds to the situation of  FIGS. 2D and 3A : after guidance and insertion of the pin and prior to locking of the pin  4   b.    
     Using a connection device  1  having two parallel openings for receiving bone pins, ensures that after fixation of a first bone pin in the bone, a second bone pin will be inserted into the bone parallel to the first bone pin. The guiding device and the cannula after all limit movement of the bone pin with respect to the connection device along the longitudinal axis of the opening. The connecting element as such guiding the second bone pin to a preferred anatomical position. 
     Providing parallel bone pins allows adjusting the distance between the connection device  1  and the patient  101 , of which the skin  101   a  is schematically indicated in  FIG. 4 . In case of for instance a swelling, as schematically indicated around bone pin  4   b , the locking devices  2   a  and  2   b  may be loosened, in this example by rotating the locking devices  2   a ,  2   b , such that the connection device  1  is again movable with respect to the bone pins  4   a ,  4   b , schematically indicated with the double arrow in  FIG. 4 . After displacement of the connection device  1  along the longitudinal axes of the bone pins, the locking devices  2   a ,  2   b  can again be tightened to lock the bone pins  4   a ,  4   b  with respect to the connection device  1 . 
     In the following figures, an external orthopaedic device, in this example in the form of a distractor  110  is explained. The external distractor  110  of this example is arranged to distract, that is gradually enlarge, the joint space width of the knee joint. The distractor  110  is thereto provided with connection devices  1   a ,  1   b  to receive and lock the pins  4   a ,  4   b  with locking devices  2 . 
     Connection device  1   a  is arranged to be connected to the tibia with two parallel bone pins  4   a , whereas connection device  1   b  is arranged to be connected to the femur using two parallel bone pins  4   b . It is noted that although it is preferred to use a connection system as shown in any of the  FIGS. 1A-1D — 4  to connect the bone pins  4   a ,  4   b  to the respective bones, this is not strictly necessary. Other suitable connection systems may be used. 
     The connection devices  1   a ,  1   b  are interconnected by an interconnecting system  120 . The interconnection system  120  comprises a central tubular member  123  of which both ends comprise receptacles  121   a,b  arranged to receive balls  140   a, b  (see for instance  FIGS. 6A and 7A ) of the connection devices  1   a, b  for forming ball joints. The receptacles  121   a, b  for the balls  140   a,b  are arranged to receive the balls  140   a, b  in a rotating manner in an unlocked position and to prevent any relative movement of the balls  140   a, b  with respect to their respective receptacles  121   a,b  in a locked position. In the unlocked position, the connection devices  1   a ,  1   b  are substantially free to rotate and angulate with respect to the central tubular member  123 , while in the locked position, the relative positions of the connections devices  1   a ,  1   b  are fixed with screw  122 . 
     With reference to  FIGS. 6A-6C , the connection device  1   b  is provided with two openings  11  which extend between a patient facing surface  12  and an outer surface  13 . Both openings  11  are oriented parallel with respect to each other. The longitudinal axes A of the openings, and thus of the inserted bone pins  4   b , therefore also extend parallel. As mentioned above, the connection device  1   b  is provided with a ball  140   b  to form a ball joint when inserted into the receptacle of the interconnecting system  120 . With specific reference to  FIG. 6B , it can be seen that the ball  140   b , in particular the centre  140   c  thereof, extends at a distance d 2  from an axis A 3  extending though (the centres of) both openings  11 . The axis A 3  is perpendicular to the longitudinal axes A of the openings. The distance d 2  is in this example determined as the minimal distance between the axis A 3  and the pivot point  140   c . As shown in  FIG. 6C , this construction creates an offset d 2  of the longitudinal axes A of the openings  11  (and thereby of the bone pins  4   b ) with respect to the pivot point  140   c .  FIG. 6C  further illustrates that the lower surface  12  of the connection device  1   b  is curved (bended) towards the patient, seen in the direction from the pivot point  140   c  towards the openings  11  in the lower surface  12 , enabling optimal intuitive guidance to anatomically preferred positions of the bone pins with proper mechanical characteristics of the whole configuration of device and bones. 
     The other connection device  1   a  is discussed in greater detail with reference to  FIGS. 7A-7D . Also this connection device  1   a  is provided with two parallel openings  11  for receiving two parallel bone pins  4   a . The openings  11  again extend between a patient facing surface  12  and an outer surface  13 . Also this connection device  1   a  is provided with a ball  140   a  to form a ball joint in combination with the receptacle  121   a  of the connecting system  120 . 
     The connection device  1   a  is however further provided with a distraction mechanism, generally indicated with  130 . The distraction mechanism comprises means to adjust the distance between the bone pins  4   a  and  4   b  of the two connection devices  1   a ,  1   b . More specifically, the distraction mechanism is arranged to gradually increase the distance between the pivot point  140   c  of the ball  140   a  and the openings  1  for the bone pins  4   a . This is achieved by cooperating threading provided on the support  131  of the ball  140   a  and rotatable ring  132  at the end of a tubular base body  134  of the connection device  1   a . Rotation of the ring  132  will adjust the distance L 1  between the base body  134  and the ball  140   a , as is visible in  FIGS. 7B and 7C . This allows adjusting the length L 1  along an adjustment axis A 4 . In this example, the adjustment axis A 4  is defined to extend through the centre  140   c  of the ball  140   a  forming the pivot point of the ball joint. 
     It will be appreciated that the adjustment of the length L 1  along the adjustment axis A 4  can take place without loosening the tension or structural integrity between the two connection devices  1   a ,  1   b , thereby keeping the joint at the preferred relative distance. In order to allow adjustment of the distance between the two sets of bone pins  4   a ,  4   b  prior to use, the connection device  1   a  comprises a body  19  which is arranged slidable along the tubular body  134  in a direction L 2 . Movement of the body  19 , which is provided with the openings  11 , can be locked using the screw  19   a.    
     In order to still allow little displacement along the adjustment axis A 4  during use of the patient, the stem  133  of the ball  140   a  is resiliently supported, in this example using a spring, in the support  131  of the ball  140   a . Support  131  and stem  133  are thus allowed to move, as indicated with the double arrow L 3  along the adjustment axis A 4 . 
     Although the ball  140   a  is allowed to move with respect to the openings  11  for receiving the bone pins  4   a , due to the distraction mechanism  130  or the sliding movement between the body  19  and the tubular body  134 , the relative orientation of the openings  11  along an axis A 3  with the respect to the adjustment axis A 4  remains unchanged. The axis A 3  extending between the two centres of the openings  11  and perpendicular to the longitudinal axes A thereof extends at a distance d 3  and parallel to the adjustment axis A 4 . The same offset d 3  is also visible in  FIG. 7D , wherein also a curved patient facing surface  12  is shown as option. 
     As is already clear from  FIG. 5 , the offset of the bone pins  4   b  in the connection device  1   b  is opposite to the direction of the offset in the connection device  1   a  with the distraction mechanism  130 . This relative orientation is further shown in  FIG. 8 , which is a view of the system of  FIGS. 1A-1D  along arrow VIII in  FIG. 5 . 
     In  FIG. 8  it can be seen that the longitudinal axes Aa, Ab of the two sets of pins  4   a ,  4   b  extend under an angle α with respect to each other. The offset d 2 , i.e. the distance between the adjustment axis A 4  (also the pivot point  140   c ) and the axis A 3  through the openings  11  of the connection device  1   b  is in the opposite direction of the offset d 3 , i.e. the distance between the axis A 3  through the openings  11  in the connection device  1   a  and the pivot point  140   c . It also shows the curvature in this case in one of the connecting devices  1   b . This configuration allows an efficient placement of the distractor  110  on the patient, with preferred anatomical location of bone pins, preventing damage to or interference with critical tissues. 
     In case of the knee joint, placement of a distractor  110  on a patient  101  is explained while referring to  FIGS. 9A-9B and 10A-10B . In a first step of fixing the distractor  110  to the patient  101  to distract the knee joint  101   b , a first bone pin  4   b  is inserted into the femur  101   c  with an orientation which is posterior (indicated with P in the cross section of  FIG. 9B  seen along the transverse plane Tp 1  of  FIG. 9A ) with respect to the coronal plane Cp. This prevents damage to and/or interference with sensible tissue. For the fixation of the first pin  4   b , a separate cannula may be used to connect the bone pin  4   b  to the bone  101   c . After fixation of the first bone pin  4   b , a connection device  1   b  of the distractor  110  is connected to the bone pin  4   b . The connection device  1   b  is then aligned such that the axis A 3  between the openings  11  is aligned to be substantially parallel to the longitudinal axis Af of the femur  101   c . The connection device  1   b  is then used as drill guide, preferably using the system as shown in  FIGS. 1A-1D-4 , to insert a second bone pin  4   b  parallel to the first bone pin  4   b  at the location of the transverse plane Tp 2 . Bone pins  4   b  then extend parallel and perpendicular to the longitudinal axis Af of the femur  101   c.    
     As the interconnecting system  120  of the distractor  110  is in the unlocked position, the connection device  1   a  provided with the distraction mechanism  130  is freely movable with respect to the already fixed connection device  1   b . As it is important that the adjustment axis A 4  of the distraction mechanism  130  is parallel to the longitudinal axis At of the tibia  101   d , ( FIGS. 10A-10B ) the connection device  1   a  is arranged such that the adjustment axis A 4  is parallel to said longitudinal axis At of the tibia  10   d . This can be done by aligning the axis A 3  through the openings  11  in the connection device  1   a  to be parallel to the longitudinal axis At of the tibia  101   d.    
     With reference to  FIGS. 10A and 10B , the trajectories A of the bone pins  4   a  are chosen to be anterior (indicated with Ant in the transverse section of  FIG. 10B  along plane Tp 1  in  FIG. 10A ) with respect to the coronal plane Cp. This prevents damage to and/or disturbance of sensible tissue. It is hereby noted that the configuration of the distractor  110  and more in particular the offsets d 2  and d 3  as shown in  FIG. 8  automatically forces the posterior placement of the bone pins  4   b  in the femur  101   c  and the anterior placement of the bone pins  4   a  in the tibia  101  (intuitive guiding of optimal anatomical positions of bone pins by the device). The bone pins  4   a  are inserted into the bone  101   d  using the openings  11  of the connection device  1   a  as drill guides, preferably using the method as shown in  FIGS. 1A-1D-4 . 
     After correct fixation of the two connection devices  1   a  and  1   b  at preferred distance from the bone/skin enabled by parallel pins), the interconnection system  120  can be moved to the locked position, thereby fixing the relative positions of the connection devices  1   a ,  1   b.    
     It is possible to use the distractor  110  as an unilateral distractor. It is however also possible to use the distractor in a bilateral configuration. For that case, a second distractor having a configuration which is mirror symmetrical with respect to the sagittal plane Sg (see  FIGS. 9B and 10B ) is connected to the patient. The orientation of the bone pins  4   a ,  4   b  for the femur  101   c  and tibia  101   d  is indicated in phantom in  FIGS. 9B and 10B . This configuration is shown in  FIGS. 11A-11C , wherein  FIG. 1A  shows a transverse section in the femur  101   c .  FIG. 11B  shows a transverse section in the tibia  101   d  and  FIG. 11A-11C  is a side view of the system. 
     The present invention is not limited to the embodiment shown, but extends also to other embodiments falling within the scope of the appended claims.