Proximal phalanx and flexor digitorum longus tendon biomechanics in flexor to extensor tendon transfer

The flexor to extensor transfer of the flexor digitorum longus (FDL) tendon has been a relatively common operative procedure for the treatment of a flexible hammer toe deformity and chronic metatarsophalangeal (MTP) joint dislocation. A possible complication of using the tunnel technique rather than the tendon splitting technique is iatrogenic fracture through the drilled tunnel site. The purpose of this investigation was to study the FDL tendon and proximal phalanx dimensions in the area of the transfer procedure in order to improve preoperative planning and minimize postoperative complications. Additionally, this study investigated the force necessary to create a fracture in a predrilled proximal phalanx and attempted to elucidate a relationship between that force and the percentage of bone remaining after the drilling process.

The proximal phalanx and FDL tendon of the second, third, and fourth toes from both the right and the left foot of 14 fresh frozen cadavers were dissected, and the digit was amputated at the MTP joint. A total of 84 toes (42 right, 42 left) were obtained from 14 cadavers. The diameter of the FDL tendon was measured, and the circumference and volume were calculated. Fourteen proximal phalanges of either the right or the left foot were then drilled with a 3.5-mm drill, as is often done in a tendon transfer procedure. The 14 nondrilled bones from the contralateral foot were used as matched controls. Radiographs were then taken of the proximal phalanges, and the dimensions of the drill tunnel and remaining bone were calculated. These measurements were used to calculate the volume of the bone, the volume of the drill tunnel, and the percentage of bone remaining after the drilling process. The bones were then tested for load-to-failure using a biomechanical loading apparatus.

The average bone and tendon diameter measurements showed a gradual decrease in size from the second to the fourth digits. The bone removed by drilling the tunnel accounted for approximately 20% to 30% of the total volume of bone. Half of the bones fractured with forces between 100 and 200 N, and the majority of bones with a diameter of less than 6 mm fractured with a force of less than 100 N.

The average proximal phalanx and FDL tendon size both showed an overall decrease from the second to the fourth digit, albeit not symmetrically. The proximal phalanx diameter appeared to be the most important factor in determining the strength of the structure.

Iatrogenic fracture may occur in proximal phalanges with a diameter of bone less than 6 mm, as there may not be adequate bone strength remaining to withstand postoperative forces.