Abstract:
A three-pin skull clamp, including a U-shaped frame defined by an interconnectable base piece with a slot that receives the ratchet arm of a ratchet piece, also includes a plate movably connected to the base piece, thereby to permit inspection and cleaning of the slot which receives the ratchet arm. To increase the adjustability of the ratchet piece relative to the base piece, the base piece includes at least two pawls with offset engagement teeth, preferably with the offset equal to half the pitch of ratchet teeth located along the ratchet arm. This feature facilitates positioning of the skull clamp relative to a patient, and is particularly beneficial for radiolucent skull clamps.

Description:
RELATED APPLICATION 
       [0001]    The present application claims priority to U.S. Ser. No. 61/211,510, filed Mar. 31, 2009, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to a skull clamp for cranial fixation, and more particularly, to a radiolucent skull clamp with improved adjustability relative to a patient&#39;s head and greater capability for cleaning and inspection. 
       BACKGROUND OF THE INVENTION 
       [0003]    Supports for securing the head of a patient during surgical or radiological procedures are known in the art. Such supports are typically adjustable so the head of the patient may be secured in different positions for different radiological views, or to facilitate access to a patient&#39;s head during a surgical procedure. Such head supports typically include a base unit that mounts to the head end of a patient support table. The base unit has a horizontal cross bar that supports one or more adapters, which in turn support a skull clamp that holds the patient&#39;s head. The skull clamp and the one or more intervening adapters or other members enable operating room attendants to adjust the height, distance, and orientation of the skull clamp with respect to the end of the table, to hold the patient&#39;s head in a desired position. An exemplary support system includes a base unit, a skull clamp, and a swivel adapter. 
         [0004]    Many skull clamps on the market use a two piece U-shaped construction with one end of the U-shape holding one inwardly directed pin and the other end holding a rocker arm that supports two spaced head holding pins. The two pieces are referred to in this disclosure as the base piece and the ratchet piece. At the closed end of the U-shape, the ratchet piece has a ratchet arm that slides through a slot or passage formed in the base piece. A locking mechanism mounted to the base piece relative to the slot engages the ratchet aim to prevent it from backing out. The slot through the base piece is only accessible from either end. Because the slot is several inches long and only wide enough to accept the complementary shaped ratchet arm, visual access to the interior surfaces of the slot or to the ratchet locking mechanism is limited. Thus, the shape and the interconnection of the slot and the ratchet arm limit the ability to clean or to inspect the slot. Yet, the use of such a skull clamp in a surgical environment practically assures that fluids of one type or another, or other substances, will inevitably find their way to the inside surfaces of the slot. Thus, there is a need for a design that facilitates cleaning and inspection of the inside surfaces of the slot and the locking mechanism. 
         [0005]    Typically, head supports such as skull clamps have been fabricated from stainless steel or other metals. However, head supports made of metal components are radiopaque to x-rays, and thus produce artifacts in the x-rays taken when the skull clamp is interposed between the x-ray source and the x-ray film. These artifacts are created by the metal components, which block the path of the x-rays as they travel from the source to the film. More particularly, the areas of the x-ray film where the x-rays are blocked by the metal will be unexposed, and will appear as artifacts in the developed x-ray picture. These artifacts diminish the usefulness of the developed x-ray picture because they obscure the image of a portion of the head that normally would be viewable, absent the obscuring metal portion of the head support. 
         [0006]    For these reasons, head supports can be made from radiolucent materials that permit x-rays to pass therethrough, thereby to reduce artifacts. An example of a known radiolucent skull clamp is shown in the Day, et al. U.S. Pat. No. 5,276,927, owned by the assignee of this application. This &#39;927 patent discloses a radiolucent head support with a radiolucent skull clamp secured to the head support. When using this structure, the radiolucent material results in fewer artifacts in the developed x-ray picture. 
         [0007]    Another example of a known radiolucent skull clamp is shown in Dinkier, U.S. Pat. No. 5,537,704, also assigned to the assignee of this application. Similar to the skull clamp structure described above, it discloses a radiolucent head clamp having a U-shaped frame with a fixed head-engaging pin at one open end of the U-shape and a pair of head-engaging pins on the opposite end of the U-shape. The closed end of the U-shape of the clamp has a radiolucent rotation mechanism for adjusting the angular position of the pair of head-engaging pins and a radiolucent translation mechanism for linearly moving the pair of head-engaging pins with respect to the fixed head-engaging pin. 
         [0008]    Generally, the skull clamps currently on the market use a ratchet and pawl mechanism to control the position of the ratchet arm relative to the base piece. More particularly, ratchet teeth along the ratchet arm engage a pawl housed inside the slot of the base piece. The pawl is biased into engagement with the ratchet teeth, so that once a ratchet tooth moves past the pawl it cannot be moved in the other direction, until a biased plunger mounted on the base piece is pulled against its biasing force to move the pawl away from the ratchet teeth, thereby to allow the ratchet arm to be removed from the slot. The distance between adjacently located ratchet teeth is called the pitch. As the ratchet arm moves further into the slot, the pawl engages another ratchet tooth each time the ratchet arm travels into the slot a distance of one complete ratchet tooth, i.e., the distance of the pitch. A neurosurgical skull clamp device that uses a sliding ratchet mechanism and single pawl is described in U.S. Pat. No. 3,835,861. Each of the three above-cited patents is incorporated by reference herein, in its entirety. 
         [0009]    With this type of structure, the distance between ratchet teeth should be sufficient enough to provide robust teeth, so as to assure a strong connection for the head holding components. However, the larger the teeth, the greater the pitch (distance between two ratchet teeth), and the greater the pitch, the lower the adjustability of the ratchet piece relative to the base piece. 
         [0010]    Greater adjustability could be achieved with finer teeth. However, finer teeth would likely be more readily damaged over the course of normal use. And for some materials, including the plastic-type radiolucent materials used to make a radiolucent skull clamp, finer teeth would be structurally susceptible to breakage. 
         [0011]    Thus, while the x-ray pictures produced by radiolucent head supports have fewer artifacts, such head supports can present other problems. For instance, compared to metals, radiolucent materials are not as well suited for certain components in the adjusting mechanisms of the above described skull clamp design. If such components are made from a plastic-type radiolucent material, the clamping forces could be reduced over time, because the plastic radiolucent components can be susceptible to deformation, and can lose their initial geometry. These conditions could eventually result, over time, in a loss of dimensional precision for the clamp. As a result, the operation and fixation of the two skull clamp frame pieces, and hence the positioning of the patient&#39;s head, could become inconsistent and unsatisfactory. 
         [0012]    Thus, there is a need to assure a long-lasting, consistent and robust connection of the two pieces of a skull clamp, particularly for a skull clamp that is made from radiolucent materials. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention meets these needs by incorporating into the base piece of a skull clamp a movably connected plate, the plate being movable relative to the base piece to open up and to close off access to the slot that receives the ratchet arm of the ratchet piece of the skull clamp, thereby to allow and to facilitate inspection and cleaning of the slot. Preferably, the plate has first and second ends, and the first end is hingedly connected to the base piece by a pin. The second end mates with the base piece to close off access to the slot, when the ratchet arm is advanced into the slot. The second end is adapted to be pulled to hingedly open the plate, when the ratchet arm is not extended into the slot of the base piece. 
         [0014]    According to a more detailed aspect of the disclosure, the plate hingedly moves about the pin, and the plate cooperates with a ratchet locking mechanism to engage the ratchet teeth of the ratchet arm to lock the ratchet piece in a desired location relative to the base piece. 
         [0015]    In an alternative approach to this embodiment, which also facilitates cleaning and inspection, the plate is removably connected to the base piece by sliding it along opposing channels located near the bottom of the slot. 
         [0016]    According to yet another aspect of the invention, the disclosed skull clamp allows better control and adjustability of the ratchet arm, and requires less force to apply the clamp to the patient&#39;s head, due to the use of a multiple-pawl mechanism for engaging the ratchet teeth. The multiple-pawl mechanism includes a first pawl and a second pawl that have engagement teeth that are offset by a distance that is a fraction of the pitch distance of the ratchet teeth, such that the ratchet arm may be engaged more than once as it travels the pitch distance into the slot. Each of the first and second pawls can have one tooth, two teeth, or more. This feature is particularly advantageous when the skull clamp is made of radiolucent material. 
         [0017]    According to a first preferred embodiment of the disclosure, the plate and the pawls rotate together. Further, the plate carries a plunger (with a plunger handle and a plunger body), which cooperates with the pawls to enable them to be biased into contact with the ratchet teeth, or removed therefrom, by pulling the plunger away from the base piece and against the biasing force. This pulling allows the ratchet arm to be removed from the slot so that the two pieces disconnect. In one sense, with this approach the improved adjustability feature can also be considered to form part of the cleaning/inspecting feature, in that the offset pawls mount to the base piece so as to be rotatable about the same pin that defines the axis of hinged connection for the plate. 
         [0018]    According to still another aspect of this particular embodiment, each of the pawls includes a spring, preferably an integral spring arm, that supplies the biasing force that biases the engagement teeth of the pawl into engagement with the ratchet teeth. This biasing force also holds the plunger handle against the base piece. 
         [0019]    A second preferred embodiment of the adjustability feature contemplates pawls that are completely independent of the removable plate. For instance, opposing pawls can be located on opposite sides of the base piece, and adapted to interact with corresponding opposing sets of engagement teeth. In this second preferred embodiment, the plate slidably removes from the base piece, and the operation of the plate is completely independent of the two opposing pawls. As a further alternative to this approach, the two pawls could be on the same side of the base piece, so long as they are offset. 
         [0020]    Other features and advantages of the present invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a perspective view of a conventional three-pin skull clamp connected to a patient table, to exemplify the use and environment of one type of skull clamp to which the present invention is applicable. 
           [0022]      FIG. 2  is a disassembled perspective view of a skull clamp constructed according to a first preferred embodiment of the disclosure. 
           [0023]      FIG. 3  is an enlarged, disassembled view of a plate that hingedly connects to a base piece of a skull clamp constructed according to the first preferred embodiment of the disclosure. 
           [0024]      FIG. 4A  is a side view of the disassembled skull clamp shown in  FIGS. 2 and 3 , with the plate hingedly moved to an open position. 
           [0025]      FIG. 4B  is a side view of the disassembled skull clamp shown in  FIGS. 2 and 3 , with the plate hingedly moved to a closed position. 
           [0026]      FIG. 4C  is a side view similar to  FIGS. 4A and 4B , but with the ratchet arm extended into the slot of the base piece. 
           [0027]      FIG. 5A  is an enlarged side view of the plate and the accompanying plunger and pawl structure, wherein the ratchet arm is extended into the slot of the base piece, with the engagement teeth of the distal pawl shown in engagement with the ratchet teeth. 
           [0028]      FIG. 5B  is an enlarged side view, similar to  FIG. 5A , but with the engagement teeth of the closest pawl shown in engagement with the ratchet teeth. 
           [0029]      FIG. 5C  is a side view, similar to  FIGS. 5A and 5B , showing the offset distance between the engagement teeth of the first end second pawls. 
           [0030]      FIG. 6  is a disassembled perspective view of a skull clamp constructed in accordance with a second preferred embodiment of the disclosure, using a completely removable plate and two opposing pawls that cooperate with corresponding rows of opposing ratchet teeth. 
           [0031]      FIG. 7  is a perspective view, in cross section, of a portion of the skull clamp  112  shown in  FIG. 6 , with a pair of opposing pawls engaging opposing rows of ratchet teeth. 
           [0032]      FIGS. 8A and 8B  each show a top view, in section, of a portion of the structure shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]      FIG. 1  shows a head support system  10  for supporting a patient (not shown) via a skull clamp  12  at the end of a surgical table  14 . According to the arrangement of components shown in  FIG. 1 , in additional to the skull clamp  12 , the head support system  10  includes a base unit  16 , which includes a crossbar  17 , and first, second, and third adapters  18 ,  19 , and  20 , respectively, which operatively connect the skull clamp  12  to the base unit  16 . Those skilled in the art will readily appreciate that in some situations only one, or possibly two, adapters may be used to adequately support the patient relative to the table  14 . 
         [0034]    The skull clamp  12  includes a hub  22  with a starburst connection that connects to the uppermost adapter  20 . The skull clamp  12  of  FIG. 1  includes a base piece  24  and a ratchet piece  26  which, when connected, generally define an upwardly opening U-shape. The ratchet piece  26  supports a single pin holding structure  28  at one open end of the U-shape, while the base piece  24  supports a rocker arm  30  capable of holding two spaced skull pins at an opposite end of the U-shape. 
         [0035]      FIG. 2  shows this structure in greater detail, with base piece  24  having a slot  32  formed therein for receiving a ratchet arm  34  of the ratchet piece. The ratchet arm  34  includes the plurality of ratchet teeth  35 . The slot  32  is sized to receive the ratchet arm  34 , as shown by directional arrow  36 . When the ratchet arm  34  moves into the slot  32 , the ratchet teeth  35  engage the internal surface in the slot  32 , so as to prevent the ratchet arm  34  from being removed from the slot  32 . A plunger handle  38  mounts to the base piece  24 , and forms part of the structure within the slot  32  that cooperates with the ratchet teeth  35  to enable the ratchet arm  34  to move further into the slot  32 , but which also prevents the ratchet arm  34  from removing outwardly from slot  32 , at least until the plunger handle  38  is pulled away from the base piece  24 . The plunger handle  38  is preferably biased to a closed position, as shown in  FIG. 2 , so as to cooperate with internal structure of the base piece  24  to engage against the ratchet teeth  35 , as will be described in more detail below. 
         [0036]      FIG. 2  also shows a plate  40  which is hingedly connected to the base piece  24  via a pin  41 . The plate  40  is hingedly connected to the base piece  24  at a first end thereof  42 , and also has a second end  44  that cooperates with the structure of base piece  24 , as for instance by a snap connection, or other suitable mechanical connection, but enables the plate  40  to be pulled downwardly (downwardly as shown in  FIG. 4A ) from the base piece  24  thereby to open access into the slot  32 . Because the plunger handle  38  is secured to the plate  40 , it hingedly moves with the plate  40  with respect to base piece  24 , about the axis defined by pin  41 . 
         [0037]      FIG. 3  shows, in a disassembled condition, the components of the base piece  24  that cooperate with the plate  40 . More particularly, the plunger handle  38  connects to a plunger body  46 . The plunger body  46  includes a recess  47  to be described in greater detail with respect to  FIGS. 4A ,  4 B, and  4 C. Although not shown in detail in  FIG. 3 , the plunger body  46  cooperates with a first pawl  48 , and the first pawl  48  includes at least one engagement tooth  49  that is shaped complementary to the ratchet teeth  35 . Preferably, the at least one first engagement tooth  49  includes two spaced engagement teeth, with a pitch equal to that of the ratchet teeth  35 . Those skilled in the art will readily appreciate that additional complementary shaped engagement teeth could be used, if desired. A second pawl  50  resides adjacent to the first pawl  48 , and includes at least one second engagement tooth  51  that is also shaped complementary to the ratchet teeth  35 . Preferably, the at least one second engagement tooth  51  also comprises two spaced engagement teeth, with a pitch equal to the pitch of the ratchet teeth  35 . 
         [0038]      FIG. 4A  shows the orientation of the base piece  24  and ratchet piece  26  prior to extension of the ratchet arm  34  into the slot  32 , but with plate  40  hingedly moved to an open position, thereby to open up access to the inside of the slot  32 . As shown in  FIG. 4A , the recess  47  of the plunger body  46  holds an outermost end  52  of first pawl  48  and also an outermost end  54  of second pawl  50  (although outermost end  54  cannot be seen in  FIG. 4A ). With this arrangement, hinged movement of plate  40  with respect to base piece  24  about an axis defined by pin  41  carries the plunger handle  38  and the plunger body  46 , while at the same time pivoting the first pawl  48  and the second pawl  50  relative to the pin  41 . 
         [0039]    With plate  40  hingedly moved to the open position relative to pin  41 , the user has easy access into slot  32 , to enable relatively easy cleaning and/or inspection of the slot  32 . This enables the operator to remove debris therefrom, or to wipe the inside surfaces clean. Also, when the plate  40  is in the open position, the ratchet piece  26  cannot be inserted into the opening. 
         [0040]      FIG. 4B  shows the plate  40  in a closed position, so that the slot  32  is entirely closed off, except at its outermost ends, and the slot  32  is ready to receive the ratchet arm  34 . With the plate  40  in the closed position, the first engagement teeth  49  of first pawl  48  reside in a position so as to engage the ratchet teeth  35  of the ratchet arm  34 , as the ratchet piece  36  moves in the direction shown via directional arrow  57 . Similarly, second engagement teeth  51  are also located and oriented in a position so as to engage the ratchet teeth  35  as ratchet arm  34  moves into the slot  32 .  FIG. 4B  shows that, in this position, a spring  58 , in this embodiment an integral spring-arm, acts on first pawl  48  and on the inside surface of the plate  40  so as to hold the first engagement teeth  49  biased in an uppermost position (upper with respect to  FIG. 4B ). 
         [0041]      FIG. 4C  shows the same components in the same relative positions, except that ratchet arm  34  has been moved further within slot  32 , as shown via directional arm  60 , which is the leftward direction as shown in  FIG. 4C . As ratchet arm  34  moves into slot  32  the ratchet teeth  35  engage first engagement teeth  49 . The force of the insertion of ratchet arm  34  causes the teeth  35  to engage the first engagement teeth  49 , and thereby cause the first pawl  48  to rotate slightly in a counterclockwise direction relative to pin  41 , against the biasing force supplied by integral spring arm  58 . As one of the ratchet teeth  35  moves past the tip of one of the engagement teeth  49 , the clockwise directed bias supplied by spring  58  causes the first pawl  48  to rotate slightly in a clockwise direction relative to pin  41 , until the next of the ratchet teeth  35  moves past, whereupon first pawl  48  again moves counterclockwise with respect to pin  41  because of the force applied by the ratchet teeth  35 . This continues so long as the ratchet arm  34  moves into slot  32 , not only with respect to first pawl  48  and first engagement teeth  49 , but also with respect to second pawl  50  and second engagement teeth  51 . 
         [0042]      FIG. 5A  shows a view similar to that of  FIG. 4C , but with the plate  40  and the components that interact therewith shown in an enlarged form. Because of the enlarged view,  FIG. 5A  shows the offset relationship between first pawl  48  and second pawl  50 . More particularly,  FIG. 5A  shows the second engagement teeth  51  residing in engagement with two of the engagement teeth  35 , while the first engagement teeth  49  have moved downwardly (counterclockwise) in  FIG. 5A , due to counterclockwise rotation about pin  41  caused by leftward movement of the engagement teeth  35 . The movement of first pawl  48  is caused by the angled surface in the ratchet teeth  35 . There is a ramping, or a camming, action when the ratchet arm  34  moves into the slot  32 . 
         [0043]      FIG. 5A  also shows the pitch, designated by P, which refers to the space between the ratchet teeth  35 .  FIG. 5A  also shows the offset distance, designated by P 1 , between first engagement teeth  49  and second engagement teeth  51 . Preferably offset distance P 1  is about half the pitch P of the ratchet teeth  35 , although any one of a number of offset distances would also work, even an offset distance greater than the pitch. 
         [0044]      FIG. 5B  is very similar to  FIG. 5A , except that the ratchet arm  34  has been moved further into the slot  32 , so that the ratchet teeth  35  have caused the second engagement teeth  51  to move downwardly, counterclockwise, against the respective biasing force, while first engagement teeth  49  have moved upwardly, i.e., clockwise relative to pin  41 , into engagement with the ratchet teeth  35 . 
         [0045]      FIG. 5C  shows an enlarged, more detailed view of the view shown in  FIG. 5B , but with the first pawl  48  shown in the foreground, and the second pawl  50  shown in the background.  FIG. 5B  shows the relative positions of these two pawls  48 ,  50 , and the first engagement teeth  49  and second engagement teeth  51 , respectively, relative to the ratchet teeth  35 . Although as shown in these Figures the offset distance P 1  is about half the pitch P of the ratchet teeth  35 , those skilled in the art will readily appreciate that the offset distance may vary, so long as it is some distance less than pitch P. This multiple pawl structure, with offset engagement teeth, enhances the adjustability of the skull clamp  12  by enabling the ratchet piece  26  to be held fixed relative to base piece  24  in essentially twice as many positions. This creates the same effect as having ratchet teeth of smaller, i.e., finer size and shape. Yet this additional adjustability is achieved without reducing the size of either the ratchet teeth  25  or the engagement  49 ,  51 . Thus, the robustness of the connection is not sacrificed. This feature is particularly advantageous if the components of the skull clamp  12  are made of radiolucent material, because with such materials it is even more important to maintain a sufficient size, shape, and pitch for the ratchet teeth  35  and the engagement teeth  49 ,  51  which engage the ratchet teeth  35 . 
         [0046]    Also, because of the shape of the second end  42  relative to the ratchet teeth  35 , the plate  40  cannot be hingedly opened when the ratchet arm  34  is located inside the slot  32 . Rather, the ratchet arm  34  must first be withdrawn from the slot  32 , via manipulation of the plunger handle  38  against the biasing force applied by integral spring arm  58 . This is shown in  FIG. 4A , which shows that the profiles of the second end  42  of the plate  40  and the ratchet teeth  35  of the ratchet arm  34  overlap to prevent such hinged opening. Because of the way the plate  40  interacts with the ratchet teeth  35 , i.e., via this structural overlap, the second end  42  serves as a safety feature for skull clamp  12 . This feature is particularly helpful when the skull clamp  12  is made of radiolucent components. 
         [0047]      FIGS. 6 ,  7 , and  8  show another preferred embodiment of the disclosure with respect to the cleaning and inspection feature, and also with respect to the adjustability feature of the clamp  112 . More particularly,  FIG. 6  shows the same generally U-shaped frame defined by base piece  124  and ratchet piece  126 . Base piece  124  includes a slot  132  sized to receive a ratchet arm  134  of ratchet piece  126 . The ratchet arm  134  includes a set of ratchet teeth  135  located along at least one of its sides.  FIGS. 7 and 8  also show that ratchet arm  134  includes an opposing or opposite set of ratchet teeth  135  located on the opposite side. 
         [0048]    A plate  140  removably connects to the base piece  124  to allow an operator to either open up or close off access to the slot  132 . Preferably, opposing channels  141  formed in the base piece  124  are sized to receive the outer edges of the plate  140 , in a slidable motion. Thus, in contrast to plate  40  (shown in the other FIGS, which hingedly connects to base piece  24 ), the plate  140  shown in  FIGS. 6 ,  7 , and  8  is completely removable from base piece  124 , by sliding along opposing channels  141 . With the plate  140  removed from the channels  141 , the slot  132  is accessible, to facilitate inspection and cleaning of the inside surfaces thereof. 
         [0049]      FIG. 6  also shows a first pawl  148  with at least one first engagement tooth  149 , and a second pawl  150  with at least one second engagement tooth  151 . The first and second pawls  148 ,  150 , respectively, rotatably mount to the base piece  124  via pins  153  and  154 , which extend into openings  155  and  156  formed in the base piece  124 , respectively, and then into openings  157  and  158  formed in the first pawl  148  and the second pawl  150 , respectively. Springs  159  and  160  bias the first pawl  148  and the second pawl  150 , respectively, into engagement with the ratchet arm  134 . 
         [0050]    This can be seen more clearly in  FIG. 7 , which also shows the two opposing sets of ratchet teeth  135  on ratchet arm  134 . More specifically,  FIG. 7  shows how the at least one first engagement tooth  149  engages the ratchet teeth  135  on one side of the ratchet arm  134 , while the at least one second engagement tooth  151  engages the ratchet teeth  135  on an opposite side of the ratchet arm  134 . The springs  159  and  160  rotatably bias the first pawl  148  and the second pawl  150 , respectively, into engagement with the ratchet arm  134 , via rotational bias about axes  153 , and  154 . This relationship can be shown more clearly in  FIGS. 8A and 8B . 
         [0051]      FIG. 8  also shows the offset relationship of first pawl  148  with respect to second pawl  150 , for the embodiment shown in  FIGS. 6 ,  7 , and  8 . More particularly, pawls  148  and  150  do not reside in the same transverse plane. But as a further alternative, the first and second pawls  148 ,  150  could be aligned in the same transverse plane, but with the ratchet teeth  135  on opposite sides of the ratchet arm  134  providing the offset. 
         [0052]    Moreover, and as yet another variation of this embodiment, the first pawl  148  and second pawl  150  could be located on the same side of the base piece  124 , with the ratchet teeth  135  located on only one side of the ratchet arm  134 . Nonetheless, there may be some preference for locating the first pawl  148  and the second pawl  150  on opposite sides of the base piece  124 . 
         [0053]    More particularly, the first pawl  148  includes an outer button surface  162 , while second pawl  150  includes a similar outer button surface  163 . These pushbutton surfaces  162 ,  163  can be pushed by an operator against the force of the springs  159 ,  160 , respectively, to overcome the biasing force, and to thereby move the at least one first engagement tooth  149  and the at least one second engagement tooth  151 , respectively, from engagement with the ratchet teeth  135 , thereby to enable the ratchet arm  134  to be removed from the slot  132 . With these pushbuttons  162  and  163  located on opposite sides of the base piece  124 , it may be easier for an operator to simultaneously push two buttons at one time, to overcome the biasing force that causes the engagement teeth to engage the ratchet teeth  135  of the ratchet arm  134 . Also, by having two separate mechanical structures that need to be acted on simultaneously by the operator and on opposite sides, this structure may provide an added degree of security, in that ratchet arm  134  may be less likely to be pulled inadvertently out of slot  132 . 
         [0054]    It should be understood that these Figures and this detailed description represent several preferred embodiments which exemplify the present invention. Those skilled in the art will readily appreciate that one or more modifications could be made to the structures shown and described herein, without departing from the principles of the invention. In that respect, each of the exemplary embodiments shown and described is meant to represent merely one subset of the subject matter covered by the broader claims appended to the end of this application. And rather than the Figures and this description, it is the appended claims which set forth the scope of the exclusionary right which the applicant seeks with respect to this application.