ORIGINAL ARTICLE Superficial Versus Deep Transfer of the Posterior Tibialis Tendon Jacques L. D’Astous, MD,*† Bruce A. MacWilliams, PhD,*† Seung-Jae Kim, MS,‡ and Kent N. Bachus, PhD† Abstract: Transfer of the posterior tibialis tendon to the dorsum of the foot is a commonly performed procedure in conditions that weaken ankle dorsiflexors and evertors, resulting in equinovarus foot deformities. When transferring the tendon, surgeons have the choice of routing the tendon deep to the extensor retinaculum or superficial to it. This study compares the biomechanics of these two routing methods. Seven cadaveric lower limbs were tested by applying known forces to the transferred posterior tibialis tendon. Resultant kine- matics indicated that passing superficial to the retinaculum resulted in a significantly more efficient motion than transfer deep to the retinaculum. Key Words: posterior tibialis, retinaculum, tendon transfer (J Pediatr Orthop 2005;25:245–248) F oot drop or inability to dorsiflex the ankle above the neutral position in the swing phase of gait is seen in numerous conditions such as Charcot-Marie-Tooth disease, Duchenne muscular dystrophy, peroneal nerve palsy, and poliomyelitis. Difficulty clearing the ground in the swing phase of gait causes the patient to drag the foot or trip. Even though the posterior tibial tendon is out of phase and does not have as much excursion as the anterior tibial muscle, it has been transferred through the interosseous membrane to the dorsum of the foot with good success in the treatment of foot drop. 3,5,6,10,12,15,18 The main indication for this procedure is the presence of a foot drop with a strong posterior tibial tendon with a grade of 4/5. Although this transfer has been advocated and performed in patients with cerebral palsy, the results are less predict- able because of the spasticity of the muscles about the ankle. 1,4,7,8,10,13,14,16,17 Historically, the posterior tibial tendon was rerouted to the dorsum of the foot by going around the medial border of the tibia, but this procedure has been largely abandoned. When transferring the posterior tibial tendon to the dorsum of the foot, most surgeons prefer going through the interosseous membrane. The tendon may then be passed deep to the extensor retinaculum or superficial to the extensor reti- naculum. The proponents of routing the tendon deep to the extensor retinaculum cite the possibility of the tendon bow- stringing across the ankle joint, making shoe wear difficult. The proponents of transferring superficial to the extensor reti- naculum feel that the transferred tendon glides better in the fat of the subcutaneous tissues and that the biomechanics are more favorable as the tendon takes a more direct route to the dorsum of the foot. The reasons for wanting the most bio- mechanically efficient transfer are that a transferred tendon usually loses one grade in muscle strength and that the disease process is often progressive, resulting in further weakening of the transferred tendon. 11 Root et al reported successful tendon transfer results with the tendon being passed superficial to the extensor retinaculum, with no mention of bowstringing as a complication. 14 The aim of the current study was to quantitatively de- termine the relative efficiencies of the posterior tibial tendon transfer to the dorsum of the foot, comparing transfers with the tendon routed deep to the extensor retinaculum to transfers routed superficial to the retinaculum. METHODS Specimen Preparation Eight fresh-frozen human cadaveric specimens were ob- tained that included the foot, ankle, and at least 20 cm of the tibia and fibula as well as all soft tissues and skin. Specimens were inspected both visually and radiographically to eliminate specimens with any evidence of arthritis, previous trauma to the ankle joint, or any gross joint malformation that could inadvertently alter the results of the experiment. One specimen was discarded due to osteoarthritis, leaving seven specimens for inclusion in this study. Mean age of the donors was 48 6 6 years (range 41–56). Specimens were kept frozen at 220°C in sealed plastic bags (4 mil Polytube, Uline, Los Angeles, CA). Prior to test- ing, specimens were thawed to room temperature (20°C) and then prepared by removing all skin and musculature from the proximal portion of the tibia and fibula to approximately 10 cm from the ankle. To increase the efficiency of the potting pro- cedure, three drywall screws (Size 9, Grabber, Concord, CA) were inserted perpendicular to the exposed bone, leaving approximately 1 cm of the screw and screw head exposed. The tibia and fibula and exposed drywall screws were placed into a 9-cm-diameter, 10-cm-long cylindrical aluminum cup. Care Study conducted at the Orthopedic Biomechanics Research Laboratory, University of Utah, Salt Lake City, Utah. From *Intermountain Shriners Hospital, Salt Lake City, Utah; †University of Utah, Department of Orthopedics, Salt Lake City, Utah; and ‡University of Utah, Department of Bioengineering, Salt Lake City, Utah. None of the authors received financial support for this study. Reprints: Jacques L. D’Astous, MD, FRCS(C), Assistant Chief of Staff, Shriners Hospital for Children, Fairfax Rd. at Virginia St., Salt Lake City, UT 84103 (e-mail: jdastous@shrinenet.org). Copyright Ó 2005 by Lippincott Williams & Wilkins J Pediatr Orthop  Volume 25, Number 2, March/April 2005 245