Instrument for evaluating balance of knee joint

A knee joint balancing instrument. A first body includes a first paddle for engaging a proximal end of a tibial bone. A second body includes a second paddle for engaging a distal end of a femoral bone. A rack is attached to the first body and is moveable relative thereto in a superior-inferior direction. The second body is pivotally mounted to the rack for pivoting about an axis extending in an anterior-posterior direction. A ratchet is connected to the first body and to the rack for selectively restraining movement of the rack in the superior-inferior direction relative to the first body. A pinion is connected to the first body and engages the rack to cause movement of the rack in the superior-inferior direction relative to the first body. Tension can be induced in the medial and lateral soft tissues of the knee joint by moving the rack, the tension being evenly distributed between the medial and lateral condyles of the knee joint by the pivot mounting of the second paddle, such that an imbalanced response of the medial and lateral soft tissues to a balanced tensile load is indicated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to surgical instruments used while 
implanting orthopedic joint prostheses, and relates more particularly to 
instruments that facilitate implantation of orthopedic knee joint 
prostheses. 
2. Background Information 
Implantable orthopedic prostheses, in one form, comprise man-made 
replacements for the ends and articulating surfaces of the bones of the 
skeleton. by Such prostheses are implanted to repair or reconstruct all or 
part of an articulating skeletal joint that is functioning abnormally due 
to disease, trauma, or congenital defect. The knee joint, as a major 
weight bearing joint, is known to degenerate relatively quickly in the 
event of abnormality. Also, the knee joint plays a critical role in 
ambulation and quality of life, resulting in great demand for surgical 
correction of abnormalities. 
To facilitate their implantation, orthopedic knee prostheses have an 
associated set of specialized surgical instruments, including some that 
are useful only with a particular prosthesis design, and others that are 
more generally useful with different prostheses. In general, instruments 
are provided for cutting and shaping the distal end of the femur, the 
proximal end of the tibia, and, sometimes, the posterior side of the 
patella, to prepare those bones to receive prosthetic articulating 
surfaces. Instruments and jigs for guiding the aforementioned cutting and 
shaping operations are another important part of the instrument set. Other 
instruments are used for holding and placing the prosthesis components 
during surgery. Still another group of instruments is used in the course 
of surgery for measuring anatomical characteristics and evaluating the 
progress and accuracy of the surgical operations performed, prior to final 
implantation of the orthopedic prostheses. The use of such surgical 
instruments can be comprehended more readily with a basic understanding of 
knee joint anatomy and the principle knee prosthesis components, as 
discussed below. 
The human knee joint involves three bones: the femur, the tibia and the 
patella, each having smooth articulation surfaces arranged for 
articulation on an adjacent articulation surface of at least one other 
bone. The femur includes at its distal extremity an articulation surface 
having medial and lateral convex condyles separated posteriorly by an 
intercondylar groove running generally in the anterior-posterior 
direction, the condyles joining at the distal-anterior face of the femur 
to form a patellar surface having a shallow vertical groove as an 
extension of the intercondylar groove. The patella includes on its 
posterior face an articulation surface having a vertical ridge separating 
medial and lateral convex facets, which facets articulate against the 
patellar surface of the femur and against the medial and lateral condyles 
during flexion of the knee joint, while the vertical ridge rides within 
the intercondylar groove to prevent lateral displacement of the patella 
during flexion. The tibia includes at its proximal end an articulation 
surface having medial and lateral meniscal condyles that articulate 
against the medial and lateral condyles, respectively, of the femur. The 
mutually engaging articulation surfaces of the femur and the patella 
together form, functionally, the patello-femoral joint, and the mutually 
engaging articulation surfaces of the femur and tibia together form, 
functionally, the tibio-femoral joint, which two functional joints 
together form the anatomical knee joint. 
The femur and tibia that comprise the human knee joint are held in proper 
relationship to each other by soft tissues, i.e., non-bony tissues, that 
span the joint and are connected to the bones on each side of the joint. 
Primarily, the soft tissues that constrain and stabilize the knee joint 
are the ligaments, although the muscles and associated tendons that induce 
motion in the joint also play a role in stabilizing the joint. In order to 
preserve the proper relationship and spacing between the femur and tibia, 
it is important that the artificial articulating surfaces be located at 
approximately the same location as the natural articulating surfaces. 
Otherwise, the ligaments that stabilize the knee joint could be either too 
tight or too loose, or unbalanced between the medial and lateral sides of 
the joint, adversely affecting the kinematics of the knee, and leading to 
accelerated wear of the prosthesis. 
All or part of one or more of the articulation surfaces of the knee joint 
may fail to perform properly, requiring the defective natural articulation 
surface to be replaced with a prosthetic articulation surface provided by 
an implantable prosthesis. To accommodate defects of varying scope, while 
permitting healthy portions of the knee joint to be conserved, a range of 
types of orthopedic knee implants is available. The range extends from 
total knee prosthesis systems for replacing the entire articulation 
surface of each of the femur, tibia and patella, to less comprehensive 
systems for replacing only the tibio-femoral joint, or only one side 
(medial or lateral) of the tibiofemoral joint, or only the patello-femoral 
joint. Commonly employed orthopedic knee prostheses include components 
that fall within one of three principle categories: femoral components, 
tibial components, and patellar components. A so-called "total" knee 
prosthesis includes components from each of these categories. The femoral 
component replaces the distal end and condylar articulating surfaces of 
the femur and may include a proximal stem that is received within the 
medullary canal at the distal end of the femur. The tibial component 
replaces the proximal end and meniscal articulating surfaces of the tibia 
and may include a distal stem that is received within the medullary canal 
at the proximal end of the tibia. The patellar component replaces the 
posterior side and natural articulating surface of the patella. Sometimes, 
the patellar component is not used, and the natural articulating surface 
of the patella is allowed to articulate against the femoral component. 
The tibial component of a total knee prosthesis is configured to be 
received upon and fixed to the proximal end of the tibia. The tibia is 
prepared to receive the tibial component by resecting a portion of the 
proximal end of the tibia to leave a substantially horizontal planar bony 
plateau. Sometimes the exposed medullary canal at the proximal end of the 
tibia is also reamed to receive a stem portion of the tibial component. 
The tibial component typically includes a plate portion having an inferior 
planar surface conforming to the resected bony plateau at the proximal end 
of the femur. The plate portion may or may not include a depending stem or 
keel for receipt within a prepared tibial medullary canal. Commonly, a 
meniscal bearing insert is received atop the plate portion of the tibial 
component to provide an artificial meniscal articulating surface for 
receiving the condylar surfaces of the femoral component of the total hip 
prosthesis. The femoral condylar articulating surfaces articulate against 
the tibial meniscal articulating surface to restore motion to a defective 
knee joint. 
One known type of tibial component involves a tibial plate made of a 
bio-compatible metal such as titanium or a titanium alloy, and a meniscal 
bearing insert made of a bio-compatible polymer such as ultra-high 
molecular weight polyethylene. The tibial plate is shaped generally as a 
flat plate having a perimeter that generally conforms to the transverse 
sectional perimeter of the resected proximal tibia. The tibial plate 
includes a planar distal, or inferior, surface for engaging the resected 
proximal tibia, and a proximal, or superior, surface for engaging and 
receiving the meniscal bearing insert. The bearing insert has an inferior 
surface that engages the superior surface of the plate portion, and may 
include locking tabs or other means for fixing the bearing insert to the 
plate portion against relative movement. 
The femoral component of a total knee prosthesis is configured to be 
received upon and fixed to the distal end of the femur. The femur is 
prepared to receive the femoral component by resecting a portion of the 
distal end of the femur to remove the natural condylar articulating 
surfaces and leave a polygonal resected bone surface. The resected bone 
surface typically includes three to five intersecting planar surfaces that 
together form a generally convex, faceted distal surface that mates 
congruently with a similar concave, faceted proximal surface of the 
femoral component. Sometimes the exposed medullary canal at the distal end 
of the femur is also reamed to receive a stem portion of the femoral 
component. The femoral component typically includes a pair of smoothly 
curved, highly polished, artificial condylar articulating surfaces that 
replace the natural condyles of the femur. The condylar articulating 
surfaces are received upon and articulate against the artificial meniscal 
articulating surface of the meniscal bearing insert described above. 
Typically, the femoral component is made of a bio-compatible metal such as 
titanium, titanium alloy, or cobalt chrome alloy. 
Various instrument designs have been proposed for tensioning the ligaments 
of the knee joint during surgery, by applying a spreading force between 
the tibia and femur, so that the spacing between the femur and tibia can 
be ascertained for a given amount of tension. Typically, the 
spacing/tension relationship is measured independently on both the medial 
and lateral sides of the joint, to detect any inequality between the 
medial and lateral soft-tissue ligaments. If one side of the knee joint is 
found to be more tightly constrained than the other, the tighter side will 
be released surgically to restore balance to the knee. One disadvantage of 
prior art instruments is that it is cumbersome to apply and maintain a 
fixed amount of spreading force to the knee joint, while allowing for 
detection of unbalance. This is because the spreading force is applied 
independently to the medial and lateral sides of the joint. It would be 
advantageous to provide an instrument that would allow a selected amount 
of spreading force to be applied to the knee joint as a whole, while 
automatically distributing the force evenly between the medial and lateral 
sides of the joint, and permitting any unbalance of the knee joint to be 
readily discerned. This and other desirable advantages are provided by the 
present invention described below. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, a knee joint balancing 
instrument has a first body including a first paddle for engaging one of a 
proximal end of a tibial bone and a distal end of a femoral bone. A second 
body includes a second paddle for engaging the other of the proximal end 
of the tibial bone and the distal end of the femoral bone. A rack is 
attached to the first body and movable relative thereto in a 
superior-inferior direction. The second body is pivotally mounted to the 
rack for pivoting about an axis extending in the anterior-posterior 
direction. Ratchet means is connected to the first body and the rack for 
selectively restraining movement of the rack in the superior-inferior 
direction relative to the first body. 
It is an object of the present invention to provide an instrument for use 
in connection with the implantation of orthopedic knee joint prosthesis, 
for evaluating the tensile balance of the soft tissues of the knee joint. 
Other objects and advantages of the present invention will be apparent from 
the following descriptions of the preferred embodiment illustrated in the 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 2, a knee balancing instrument 10 is shown. A 
torque driver accessory instrument 12 is also shown in FIG. 1. A 
directional frame of reference is included in FIG. 1 to facilitate the 
following description of knee balancer 10 in terms of well-known 
anatomical directions as they would apply to knee balancer 10 in use. The 
anterior-posterior direction is indicated by the symbols A-P, the 
superior-inferior direction is indicated by the symbols S-1, and the 
medial-later direction is indicated by the symbols M-L. As shown in FIG. 
1, the medial and lateral directions indicated by M and L, respectively, 
correspond to use of knee balancer 10 on the left knee. When used on the 
right knee, the M and L directions would be reversed. 
Knee balancer 10 includes as principle components a first body 14, a rack 
16, and a second body 18. First body 14 includes an attachment block 20, a 
handle 22 extending from the inferior end of block 20 generally in the 
anterior direction, and a first paddle 24 extending from the superior end 
of block 20 generally in the posterior direction and includes a posterior 
notch 25. As preferred, attachment block 20, handle 22 and first paddle 24 
are integrally joined. A bore 26 extends through attachment block 20 in 
the superior--inferior direction and is open at the superior and inferior 
ends. A plurality of mounting pin holes 27 extend through attachment block 
20 in the anterior-posterior direction to either side of bore 26, for 
receiving mounting pins (not shown) therethrough. The mounting pins can be 
driven into the anterior side of the tibia to attach block 20 to the tibia 
in a stable, but temporary, manner. Blind holes 29 are located on the 
anterior face of attachment block and extend in the anterior-posterior 
direction for receiving an extension member and an associated alignment 
rod, described further below. Rack 16 is disposed within bore 26 in 
sliding relationship for movement in the superior-inferior direction 
relative to attachment block 20. Ratchet teeth 28 are located along a 
substantial length of the anterior side of rack 16. A passage 30, open at 
the anterior side of attachment block 20, communicates with bore 26 and 
provides access to ratchet teeth 28 of rack 16. A pawl 32, pivotally 
attached to first body 14, extends through passage 30 and engages ratchet 
teeth 28 to restrain rack 16 against movement in the inferior direction 
when so engaged. A spring (not shown) biases pawl 32.toward engagement 
with ratchet teeth 28. Pawl 32 includes a release lever 34 which, when 
depressed in the inferior direction toward handle 22, causes pawl 32 to 
pivot out of engagement with ratchet teeth 28, allowing rack 16 to move in 
the inferior direction. Rack 16 includes rack gear teeth 36 extending 
along a substantial length of rack 16 on the right side of rack 16. A 
passage 38, open at the right side of attachment block 20, communicates 
with bore 26 and provides access to rack gear teeth 36 of rack 16. A 
pinion gear 40, mounted to first body 14 for rotation about an axis A 
extending in the anterior-posterior direction, includes pinion gear teeth 
42 that extend through passage 38 and engage rack gear teeth 36. Pinion 
gear 40 includes a driving interface 44 exposed on the anterior side of 
first body 14 for receiving the torque driver 12 in rotary driving 
engagement. Torque driver 12 is used to rotate pinion gear 40 in the 
appropriate direction to move rack 16 in the superior-inferior direction 
relative to first body 14. Second body 18 is pivotally mounted to the 
superior end of rack 16 by pivot pin 46 for pivoting movement about an 
axis B extending in the anterior-posterior direction. A spring-rod 48, 
extending in the medial-lateral direction and r mounted at each end to 
second body 18, has a middle portion that can engage the superior end of 
rack 16. As second body 18 is pivoted on pivot pin 46, the middle portion 
of spring-rod 48 engages the superior end of rack 16 and is deflected 
elastically. The spring-rod 48 tends to bias second body 18 toward a 
neutral pivot orientation relative to rack 16. A second paddle 50, 
integral with second body 18 and extending generally in the posterior 
direction, lies parallel to first paddle 24 when second body 18 is in the 
neutral pivot orientation and includes a posterior notch 51. An angle 
reference plate 52, affixed to second body 18, has an index line 53 that 
extends in the superior direction when second body 18 is in the neutral 
pivot orientation. An indicator pin 54, affixed to rack 16, is offset 
anteriorly from angle reference plate 52 and extends in the superior 
direction, parallel to the longitudinal axis of rack 16. A rack handle 56, 
affixed to the inferior end of rack 16, extends generally in the anterior 
direction parallel to handle 22 of first body 14 to prevent rack 16 from 
escaping bore 26, and to facilitate manual movement of rack 16 relative to 
first body 14 when pawl 32 is disengaged from ratchet teeth 28. 
Referring specifically to FIG. 2, knee balancer 10 is shown in a first 
orientation in which first paddle 24 and second paddle 50 are drawn 
together such that the spacing in the superior-inferior direction between 
the inferior surface 58 of first paddle 24 and the superior surface 60 of 
second paddle 50 is at a minimum. As preferred, the minimum spacing is 
about 9 mm. 
Referring specifically to FIG. 3, knee balancer 10 is shown in a second 
orientation in which first paddle 24 and second paddle 50 are spaced apart 
such that the spacing in the superior-inferior direction between the 
inferior surface 58 of first paddle 24 and the superior surface 60 of 
second paddle 50 is at a maximum. As preferred, the maximum spacing is 
about 84 mm. 
With reference to FIGS. 4, 5 and 6, second paddle 50 is shown in the 
neutral orientation, pivoted left about 10.degree., and pivoted right 
about 10.degree., respectively. As shown in FIGS. 5 and 6 especially, any 
deviation of second paddle 50 relative to the neutral position shown in 
FIG. 4 is discernable by the physician by observing the misalignment of 
index line 53 of angle reference plate 52 relative to indicator pin 54. 
The direction of tilt is readily observable. 
As preferred, the knee balancer 10 described above is used to determine the 
state of balance of a knee joint under tension. Using knee balancer 10, a 
selected load is applied to the knee joint along the mechanical axis of 
the knee joint. The pivoted mounting of second paddle 50 provides even 
distribution of the load across the knee joint, such that any imbalance of 
soft tissue constraints can be detected. This results in a clear and 
accurate indication of the state of balance of the knee joint, and of the 
soft tissues that require release to achieve balance. 
Once the tibia has been resected to the desired plane, the knee joint is 
placed in flexion. The knee balancer 10 is inserted into the knee joint 
such that the inferior surface 58 of the first paddle 24 rests on the 
resected bony plateau of the proximal tibia. As an option, first body 14 
can be secured in place on the tibia by way of mounting pins received 
through mounting pin holes 27 and driven into the tibial bone. The second 
paddle 50 is moved in the superior direction, along with rack 16, relative 
to first paddle 24 and first body 14, until the superior surface 60 of 
second paddle 50 engages the distal condyles of the femur. The second 
paddle is then moved further in the superior direction by means of the 
torque driver 12 applied to pinion driving interface 44. The spacing 
between the first paddle 24 and second paddle 50 is increased, placing the 
soft tissues of the knee joint in tension, until the desired amount of 
tension is achieved, as indicated by the torque measured by the torque 
driver 12. The joint gap is then measured, and the balance of the knee is 
evaluated by observing the orientation of the index line 53 of angle 
reference plate 52 relative to the indicator pin 54. If the index line 53 
of angle reference plate 52 and indicator pin 54 are aligned, then the 
soft tissue of the knee joint is balanced. If angle reference plate 52 is 
displaced at an angle to the left or right of indicator pin 54, then 
surgical release of the soft tissue on the side to which angle reference 
plate 52 is displaced will be necessary to restore balance to the knee 
joint. Surgical release is effected until balance is indicated by index 
line 53 of angle reference plate 52 becoming aligned with indicator pin 
54. After soft tissue release, the joint tension should again be checked 
with the torque driver 12 and adjusted to the desired level, if necessary. 
The joint gap should then be measured again. Tension is then released by 
depressing the pawl handle 34, disengaging pawl 32 from ratchet teeth 28 
and allowing second paddle 50 and rack 16 to move freely toward first 
paddle 24 and first body 14. The knee joint should then be placed in 
extension and the joint gap and balance should be checked again according 
to the steps described above. The joint gap should be the same in both 
flexion and extension. Once the joint gap and knee balance are determined 
to be satisfactory, the alignment of the knee joint should be checked. 
This is accomplished by mounting an extension member (not shown) to the 
anterior face of alignment block 20 such that the extension member extends 
in the anterior-posterior direction. An alignment-rod (not shown) extends 
superiorly and inferiorly from the extension member, 61 Pins of the 
extension member are received in a removable fit within holes 29 of 
alignment block 20. While the extension member and the alignment rod are 
so mounted, the surgeon observes whether the rod is aligned with the hip, 
knee and ankle joints, all three of which should be aligned for proper 
kinematics of the joints. 
The present invention has been illustrated and described with particularity 
in terms of a preferred embodiment. Nevertheless, it should be understood 
that no limitation of the scope of the invention is intended thereby. The 
scope of the invention is defined by the claims appended hereto. It should 
also be understood that variations of the particular embodiments described 
herein incorporating the principles of the present invention will occur to 
those of ordinary skill in the art and yet be within the scope of the 
appended claims.