Why Is Anybody Still Doing Flexor Digitorum Longus Tendon Transfers?

When planning surgical repair of the adult-acquired flatfoot secondary to rupture of the posterior tibial tendon, transfer of the flexor digitorum longus tendon to the navicular has always been a popular solution. From an anatomic perspective, this surgical solution seems to make perfect sense.

The flexor digitorum longus lies immediately adjacent to the deficient or ruptured posterior tibial tendon. When attached to the navicular, the flexor digitorum longus now appears to have replaced the posterior tibial tendon to provide essential inversion moment, or supination force to the hindfoot. 

While tradition and intuition justify the flexor digitorum longus tendon transfer, the truth is that this procedure has little mechanical effect on the flatfoot deformity. Even more important is recognizing that multiple research studies have shown that this tendon transfer may actually weaken the inversion power of the flexor digitorum longus in comparison to leaving the tendon alone and not doing a transfer at all.

Goldner and colleagues first proposed the concept of transferring the flexor digitorum longus tendon in the treatment of posterior tibial tendon dysfunction (PTTD) in 1974 and then Mann popularized it in 1983.^1,2 While early results of this procedure appeared favorable, it soon became apparent that isolated tendon transfer of the flexor digitorum longus would fail in most cases.³

Despite the dismal results of transferring the flexor digitorum longus to the navicular in hopes of replacing the ruptured posterior tibial tendon, surgeons continued to embrace this concept as an adjunctive procedure to other approaches in adult-acquired flatfoot surgical correction, including a medializing calcaneal osteotomy and lateral column lengthening.^4,5

It is quite remarkable that a procedure that by itself showed no benefit in adult-acquired flatfoot surgery continued to be a standard of operative care for over 40 years despite the fact that biomechanical studies had previously demonstrated the futility of substituting the flexor digitorum longus for the posterior tibial tendon. In 1976, Murray demonstrated that the flexor digitorum longus could only generate half the inversion torque of the posterior tibial tendon.⁶ In terms of counteracting the deforming force of the peroneus brevis, Murray and coworkers showed that the flexor hallucis longus tendon has a twofold greater strength than the flexor digitorum longus. Indeed, others have taken notice and implemented the flexor hallucis longus tendon transfer in favor of the flexor digitorum longus tendon transfer in PTTD surgery.⁷

Rather than looking at pure tendon strength, studies of moment arm and correction of foot alignment with the flexor digitorum longus tendon transfer are most convincing of the futility of this procedure. Consider the findings from four landmark studies.

1. Researchers at the University of Utah demonstrated that the flexor digitorum longus transfer to the navicular reduced the capacity of this tendon to invert the hindfoot by 36 percent in comparison to leaving the tendon in its native state.⁸
2. Researchers at Lehigh University used multi-segmented foot modeling to assess the effectiveness of the medial displacement calcaneal osteotomy to correct the adult-acquired flatfoot. This study showed no improvement in abnormal loading or deforming joint moments in the medial arch with flexor digitorum longus tendon transfer.⁹
3. Researchers at the University of Washington used a cadaveric robotic gait simulator to study the effects of the flexor digitorum longus tendon transfer on a flatfoot model. No improvement of foot alignment or function occurred with this procedure.¹⁰
4. Researchers at Virginia Commonwealth University studied cadaveric specimens and determined that the flexor digitorum longus tendon transfer actually increased loads on the medial arch and medial heel, exacerbating the deforming forces of the adult-acquired flatfoot.¹¹

These studies clearly demonstrate that the flexor digitorum longus tendon transfer has no positive effect and may potentially have a negative effect from a mechanical standpoint in correcting the adult-acquired flatfoot deformity. These are independent investigations conducted at prestigious institutions around the United States. All of the studies were peer reviewed and published in reputable scientific journals.

Despite this published knowledge, the flexor digitorum longus tendon transfer continues to be practiced and taught within the podiatric and orthopedic profession. This week at the annual meeting of the American College of Foot and Ankle Surgeons (ACFAS), researchers presented the flexor digitorum longus transfer as a viable adjunctive procedure to treat the adult- acquired flatfoot.

It should be noted that there is limited biomechanics content at this year’s ACFAS meeting.

References

1. Goldner JL, Keats PK, Bassett FH III, Clippinger FM. Progressive talipes equinovalgus due to trauma or degeneration of the posterior tibial tendon and medial plantar ligaments. Orthop Clin North Am. 1974; 5(1):39-51. 

2. Mann RA. Acquired flatfoot in adults. Clin Orthop. 1983; 181:46-51.
3. Mlchelson J, Conti S, Jahss M. Survivorship analysis of tendon transfer surgery for posterior tibial tendon rupture. Orthop Trans. 1992; 16(1):30.
4. Myerson MS, Corrigan J. Treatment of posterior tibial tendon dysfunction with flexor digitorum longus tendon transfer and calcaneal osteotomy. Orthopedics. 1996; 19(5):383-388.
5. Hiller L, Pinney SJ. Surgical treatment of acquired flatfoot deformity: what is the state of practice among academic foot and ankle surgeons in 2002? Foot Ankle Int. 2003; 24(9):701-705.
6. Murray MP, Guten GN, Baldwin JM, Gardner GM. A comparison of plantar flexion torque with and without the triceps surae. Acta Orthop Scand. 1976; 47(1):122-124.
7. Sammarco GJ, Hockenbury RT. Treatment of stage II posterior tibial tendon dysfunction with flexor hallucis longus transfer and medial displacement calcaneal osteotomy. Foot Ankle Int. 2001; 22(4):305-312.
8. Hui HE, Beals TC, Brown NA. Influence of tendon transfer site on moment arms of the flexor digitorum longus muscle. Foot Ankle Int. 2007; 28(4):441-7.
9. Arangio GA, Salathe EP. A biomechanical analysis of posterior tibial tendon dysfunction, medial displacement calcaneal osteotomy and flexor digitorum longus tendon transfer in adult acquired flatfoot. Clin Biomech. 2009; 24(4):385-390.
10. Vaudreuil NJ, Ledoux WR, Roush GC, Whittaker EC, Sangeorzan BJ.  Comparison of transfer sites for flexor digitorum longus in a cadaveric adult acquired flatfoot model. J Orthop Res. 2014; 32(1):102–109.
11. Spratley EM, Arnold JM, Owen MS, et al. Plantar forces in flexor halluxcis longus versus flexor digitorum longus transfer in adult acquired flatfoot deformity. Foot Ankle Int. 
2013; 34(9):1286–1293