Despite many advances, and decades of clinical practice and research, there is still room for improvement in the field of flexor tendon repairs. Not all patients regain good movement and function and there is a significant post-repair rupture rate. Achieving the balance between early mobilisation of the tendon to regain optimal function and protecting the repair is difficult and requires significant therapy input and patient compliance. Any advance resulting in stronger tendon repairs and less prescriptive rehabilitation is likely to improve outcome and save healthcare resources.
Whilst implants for tendon fixation have been developed and marketed1 none have proved overall superior to the best of the suture techniques.2,3 Concerns have been raised about the additional tendon exposure and dissection required to implant the devices.
Shape memory metals were discovered in the 1930s and further developed in the 1960s by the Naval Ordnance Laboratory. Commonly nickel titanium alloys, they exist in two crystalline forms and transform from one to another on heating and cooling. Not only are these metals very flexible, but if such an alloy is deformed at low temperature, on heating it resumes its previous shape.
The dynamic properties of these metals have been exploited commercially in many forms, including use in dental braces, cardiac stents, and orthopaedically, as bone anchors, staples and meniscal darts.
Hirpara, Sullivan and O’Sullivan have devised a barbed implant for flexor tendon repairs, using a shape memory metal, Nitinol. Barbs are cut in the edge of a length of nitinol tubing and manually primed, so as to grip the tendon once inserted and resist pull-out. Unlike previous tendon fixation devices, which required significant tendon exposure and dissection for implantation, Hirpara’s device is inserted through the cut tendon ends with little further exposure. In vitro testing on porcine flexor tendons shows promising early results with comparable strength to a Savage six strand repair.
Many questions remain to be answered. Will the devices function as well in vivo or fail on cyclical loading? Will any bio-incompatibility compromise fixation strength? Will a device of adequate fixation strength occupy too great a percentage of the cross sectional area of the tendon to allow good healing? Will the stiffness of the implant increase the work of flexion of the tendon? If the device fails or its insertion is problematic, is there an acceptable salvage option for the tendon?
We await the results of clinical trials with anticipation. If they do prove favourable, will the improved patient outcomes and healthcare savings justify the additional cost of an implant over a suture?
And should the device prove superior to standard suture techniques, one undesirable consequence is the continued deskilling of surgeons in basic suture repairs which may be necessary again in the future, should shape memory metal implants become unavailable or unaffordable.
1. Teno Fix. Ortheon Medical, Columbus, Ohio, USA. firstname.lastname@example.org.
2. Su B, Solomons M, Barrow A, et al. Device for Zone-II flexor tendon repair: a multicenter, randomized, blinded clinical trial. J Bone Joint Surg [Am] 2005;87-A:923-35.
3. Wolfe S, Willis A, Campbell, et al. Biomechanic comparison of the Teno Fix tendon repair device with the cruciate and modified Kessler techniques. J Hand Surg [Am] 2007;32:356-66.
Fullilove S, MBBS, MA(Oxon), FRCS(Orth)
Derriford Hospital, Plymouth, Devon, United Kingdom