Relationship of Sagittal Curvature of Condyles to ROM

Abduction-adduction at the PIP joint is constrained by a bi-condylar configuration on the articulating surface and the collateral ligament support. Only flexion extension, similar to a mechanical hinge joint, is possible in general as the joint motion. The hand digits exhibit convergent arcs during flexion towards the palm. Their axes of rotation vary and are not perpendicular to the longitudinal axes of phalangeal bones, except in the long finger [49, 50], due to asymmetry at the radial and ulnar condyles. The PIP joint is at neutral position (0°) when the middle phalanx is straightly aligned with the longitudinal axis of the proximal phalanx. The active ROM of the PIP joint ranges from 108° to 113° with an average of 110° [51]. Hyperextension of a joint is possible but limited to -5°, constrained by the volar plate and its check-reins ligament at the palmar aspect. The functional ROM was found to be 36°-86° for a selection of most common prehensile activities from daily living [52]. Passive joint motion is larger in extension but about the same in flexion to the active range.

To achieve a joint flexion over 90°, the head of the proximal phalanx is slightly offset from the longitudinal axis of the bone shaft. Figure 9.4 shows the motion from a PIP joint where the middle phalanx is indicated at its relative positions from full extension to flexion forming a maximum ROM of 110°. The arcs of coverage of the articulating surfaces on the head of proximal phalanx and the base of middle phalanx are denoted 'ah' and 'ab' respectively. Based on this arrangement, the dimension of ah can be determined as not smaller than the sum of (ab plus ROM of 110°). These two dimensions were measured from the appearance of the layer of underlying calcified cartilage right above the subchondral bone. Measurements show mean values of 201.5° and 76.3° of the mixed group of the angle arc of coverage on the head of proximal phalanx and base of middle phalanx respectively. The measurements agree with the assumed condition of ah > ab + ROM (201.5° > 76.3° + 110°) under full joint contact. The dimensions 'ah' and 'ab' of the arc of coverage of the articulating surfaces are applied to the design of the artificial joint surfaces on the proximal and distal components respectively. Increasing the arc of coverage on the distal component denoted by dimension 'ab' could enhance the implant stability in resisting joint subluxation on both the dorsal and palmar directions accordingly, but possible ROM would be lessened as a consequence.

Fig. 9.4 Major arc dimensions on articulating surfaces for ROM
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