The Relationship of the Lapidus Intramedullary Nail to Neurologic and Tendinous Structures in the Foot: An Anatomic Study

CATEGORY: Bunion, Midfoot/Forefoot INTRODUCTION/PURPOSE: The lapidus procedure is a longstanding operation performed for the treatment of hallux valgus deformity with a concomitant hypermobile first ray. Orthopaedic surgeons have a myriad of options to choose from in performing the lapidus procedure. The implantation of orthopaedic implants comes with the risk of iatrogenic injury to surrounding anatomy. Several cadaveric studies in the humerus and femur have described potential neurovascular structures at risk during placement of intramedullary nail systems. The purpose of this study was to determine the proximity of nail insertion and interlocking mechanisms in the Lapidus Phantom Intramedullary Nail System (Paragon 28, Inc.) to neurologic and tendinous structures in the foot. METHODS: A titanium intramedullary nail was inserted from the first metatarsal to the medial cuneiform spanning the first tarsometatarsal joint in 10 fresh-frozen cadaver feet. K-wires were inserted in the proximal lateral, proximal medial, and distal medial-to-lateral interlock screw paths. The tibialis anterior tendon, extensor hallucis longus tendon, and superficial peroneal nerve were carefully dissected and exposed, and the distance from each of these anatomic landmarks were then measured and recorded from four different aspects of the nail: Proximal lateral interlocking screw path, proximal medial interlocking screw path, nail insertion, distal interlocking screw path. Distances were averaged, ranges were determined. RESULTS: The tibialis anterior tendon was in closest proximity to the proximal medial interlock K-wire with an average distance of 0.4 mm. The proximal medial interlock bisected the tibialis anterior tendon in three of the specimens. The extensor hallucis longus tendon was in closest proximity to the nail insertion with an average distance of 1.2 mm. A branch of the superficial peroneal nerve was in closest proximity to the distal interlock K-wire with an average distance of 7.5 mm, however, the nerve came in direct contact with the proximal medial interlock K-wire in two of the specimens. CONCLUSION: The Lapidus intramedullary nail’s proximal medial interlock screw poses the greatest threat to the tibialis anterior tendon, with the distance from the tendon to the interlock K-wire being 3 mm or less in all specimens tested. The extensor hallucis longus tendon is at risk of injury with insertion of the nail. Medial to lateral interlocking poses the greatest danger to a branch of the superficial peroneal nerve. Blunt dissection should be performed using this system with a path to bone before instrumentation to reduce the risk of nerve and tendon injury in the foot.