The Effect of Progressive Extensor Retinaculum Excision on Wrist Biomechanics and Bowstringing.

PURPOSE: To quantify the effect of the extensor retinaculum in preventing bowstringing and extensor lag. METHODS: The extensor carpi radialis brevis (ECRB) and extensor digitorum communis (EDC) tendons to the middle finger were isolated in 6 human cadaveric specimens. Extensor tendon excursion and bowstringing were measured as the extensor retinaculum was serially excised. RESULTS: For the second dorsal compartment, extensor lag averaged 7° when the entire retinaculum over the ECRB was excised. Bowstringing did increase with sequential resections and was greater for distal resections than for proximal resections. When the entire retinaculum over the ECRB was removed, bowstringing averaged 12 mm. For the fourth dorsal compartment, excision of the distal retinaculum resulted in more bowstringing and extensor lag than excision of the proximal retinaculum. When the proximal two-thirds was excised, EDC extensor lag averaged 12° and bowstringing averaged 9 mm; with the distal two-thirds excised, extensor lag averaged 18° and bowstringing averaged 14 mm. Complete retinaculum excision resulted in EDC bowstringing of about 61 mm and extensor lag of 80°. CONCLUSIONS: Resection of the entire extensor retinaculum over the second dorsal compartment results in minimal extensor lag and minimal bowstringing. Resection of the entire retinaculum over the fourth dorsal compartment results in massive extensor lag and bowstringing and should be avoided. The distal portion of the retinaculum is most important in preventing extensor lag and bowstringing for the fourth compartment. CLINICAL RELEVANCE: The findings inform the surgeon's handling of the extensor retinaculum during procedures on the dorsum of the wrist, especially when portions are to be excised, transposed, or repaired.

The effect of posterior decompressive procedures on segmental range of motion after cervical total disc arthroplasty.

STUDY DESIGN: We quantified the segmental biomechanics of a cervical total disc replacement (TDR) before and after progressive posterior decompression. We hypothesized that posterior decompressive procedures would not significantly increase range of motion (ROM) at the index TDR level. OBJECTIVE: To quantify the kinematics of a cervical total disc replacement (TDR) before and after posterior cervical decompression. SUMMARY OF BACKGROUND DATA: A reported yet unaddressed issue is the potential for the development of same-segment disease after implantation of a cervical TDR and the implications of same-segment posterior decompression on TDR mechanics. METHODS: Eight human cadaveric cervical spines C3-C7 were tested in flexion-extension, lateral bending, and axial rotation while intact, after C5-C6 TDR, C5-C6 unilateral foraminotomy, C5-C6 bilateral foraminotomies, and after C5 laminectomy in combination with the bilateral foraminotomies. Moment versus angular motion curves were obtained for each testing step, and the load-displacement data were analyzed to determine the range of angular motion for each step. RESULTS: Unilateral foraminotomy did not result in a statistically significant increase in flexion-extension ROM, and did not increase the ROM to a degree greater than normal. Although bilateral foraminotomies did increase flexion-extension ROM, motion remained within a physiological range. A full laminectomy added to the bilateral foraminotomies significantly increased ROM and was also associated with distortion of the load-displacement curves. CONCLUSION: With respect to segmental biomechanics as demonstrated, we think that for same-segment disease, a unilateral foraminotomy can be performed safely. However, the impact of in vivo conditions was not accounted for in this model, and it is possible that cyclical loading and other physiological stresses on such a construct may affect the behavior and lifespan of the implant in a way that cannot be predicted by a biomechanical study. Bilateral foraminotomies would require close observation and additional clinical follow-up, whereas complete laminectomy combined with bilateral foraminotomies should be avoided after TDR given the significant changes in kinematics. In addition, future disc replacement designs may need to account for changes after posterior decompression for same-segment disease. LEVEL OF EVIDENCE: N/A.