Using a reverse engineering capability to quantify the factors that control the rigid body mechanics of the wrist, a mathematical forward animation capability and model of wrist motion that allows the carpus to move under its own rules is being developed. This characterises the isometric connections, from which was developed the Stable Central Column Theory of Carpal Mechanics - which incorporates the Law of Rules Based Motion. This work has now advanced to the ability to reapply the extracted rules to allow rules-based rigid body reanimation of an individual wrist. As each wrist is unique, there is a given reality that each reanimation must be based on an individual wrist's unique rules, and the aspiration to create a standard or normal wrist is unrealistic. Using In the preliminary application of this biomechanics environment, by using the reverse engineering / forward reanimation process, wrist motion can be recreated - based purely on the unique rules, extracted from individual wrists. Instability of the proximal scaphoid was evident in several of the animations, and there was confirmation that the spatial attachment points of the isometric constraints are very exacting. The actual attachment and specific morphology of the carpal bones varied between individual wrists. Using a reverse engineering and then forward reanimation process, we have been able to recreate wrist motion using the rigid body mechanics based on the Law of Rules Based Motion. Further work is required, but the potential to apply “what if” virtual surgery options to an individual injured wrist and more precisely characterise and test solutions to wrist dysfunction are becoming realised.
This study has developed a unifying theory of carpal motion based on computer derived isometric constraints which guides the movement of particular bones. This extends the previously reported concept of rules based animation which proposes that resultant motion is a net interplay of bone shape, isometric constraints, bone interaction, and applied load The positional relationship between bones of the proximal row and the radius at extremes of motion was assessed to identify isometric constraints, based on a computer derived analysis rather than by observation of carpal bone motion or ligamentous anatomy. Using 3-D surface rendering software, models were created from the CT scan data of 10 normal wrists taken in extremes of radial and ulna deviation as well as flexion and extension. Virtual lines were identified between specific points of the lunate and radius which corresponded to an isometric constraint through range. Similar pairs of points were found at the trapezium and scaphoid and dorsally at the scapho-lunate joint. There was a clear discrepancy (p <
.05) between those areas (typically either volar or dorsal depending on the bones) which remain isometric and those which did not and this corresponded to previous documented anatomical structures. Variability in the pattern of isometric lines correlated with variation in scaphoid motion, thus providing a correlation with previous carpal motion observations. The Carpus can be seen to function as a stable central column (lunate/capitate/hamate/trapezoid/trapezium), with a supporting lateral column (scaphoid). This functions more as a “crossed four bar linkage” than the traditionally described “slider crank”. On the medial side of the central column, the triquetrum acts principally as an ulna translation restraint. The “trapezoid” shaped trapezoid places the trapezium anterior to the transverse plane of the radius and ulna, and thus rotates the principal axis of the central column to correspond to that used in the “Dart Thrower Motion”. This model provides a unifying theory for understanding normal and abnormal wrist motion based on isometric constraints and more broadly rules based motion. The characterisation of isometric constraints within the proximal carpal row has allowed the quantitative analysis of carpal dynamics, which has as its core, a stable central carpal column – with a lateral column stabiliser, and medial column translation restraint key to safe administration of anaesthetic in the upright position.
We tested the hypothesis that it is possible to accelerate fracture healing by changing the mechanical environment used in current methods i.e., from initial rigidity or micromovement followed by dynamisation to initial macromovement followed by rigidity (micro-movement). It is accepted that callus formation requires movement at the fracture site and this callus response is limited to the first few weeks after fracture. Logically, early macromovement at the fracture site would be beneficial for callus formation. Additional callus is not produced by further movement. Indeed, it may be counter-productive, just as continuing movement around two ends of a wooden stick bonded with glue will retard and even prevent “union”. We postulate that continuing movement at the fracture site after the callus response has ceased will also delay union. As a result, rigidity rather than dynamisation is required in the later stage of fracture healing. After testing an animal model, we built an external fixator which allowed 5 mm of axial movement without “self-locking” and could be compressed at a later date in order to prevent further movement. A trial containing 15 patients with unilateral tibial shaft fractures (closed or grade 1 open) was undertaken after permission was obtained from the Helsinki Ethical Committee. So far, 13 patients have been entered into the trial. They have completed therapy and are at least one year post-fracture (12 months to 22 months). Age range is from 20 to 49. The group is composed of nine males and one female. Under general anaesthetic, an external fixator was applied and the fracture reduced. The patients started ankle exercises (active and passive) the following day, with as much weight-bearing on the fractured leg as possible on the day after. The patients were seen every two weeks and AP and lateral radiographs were taken. The fracture was compressed two to six weeks later. The percentage of body weight that the patient was able to tolerate through the fractured limb was measured by using the scales of Meggit’s step test. The fixators were removed when there was radiographic union and the patient could take at least 80% of body weight through the fractured limb. Mean time duration up to removal of the fixator was 10.8 weeks (range 7 to 15.4 weeks). We conclude that it is possible to increase the speed of bone healing by changing the mechanical environment to initial macromovement followed by elimination of movement.
