The Response of the Flexor Digitorum Longus and Posterior Tibial Muscles to Tendon Transfer and Calcaneal Osteotomy for Stage II Posterior Tibial Tendon Dysfunction

Abstract

Background: The purpose of this prospective study was to evaluate the response of the flexor digitorum longus (FDL) and posterior tibial (PT) muscles to FDL tendon transfer and medial displacement calcaneal osteotomy for stage II posterior tibial tendon dysfunction (PTTD). Methods: Twelve patients were divided into two groups, depending on whether the PTtendon was excised(Excised Tendon Group) or left intact (Intact Tendon Group). The muscle volumes of the FDL and PT muscles in both legs were measured and compared, using cross-sectional area (CSA) analysis of preoperative and postoperative MRI. Results: Preoperatively, there was an average 11% reduction in the PT muscle volume and a 17% increase in the FDL muscle volume from the normal contralateral side in both groups. One year after surgery (average 13.4 months) in both groups, the FDL muscle volume had increased by an average of 27% and the PT muscle volume had decreased by 23% compared to the contralateral normal side. The FDL volume increased by 44% in the Excised Tendon Group compared to 11% in the Intact Tendon Group. The PT muscle volumes were not assessed in the Excised Tendon Group because all PT muscle had been replaced by fatty infiltration. The PT volumes in the Intact Tendon Group decreased further from a 6% reduction preoperatively to a 23% reduction postoperatively compared to the normal contralateral side. The American Orthopaedic Foot and Ankle Society (AOFAS) hindfoot scores increased from 50 preoperatively to 88 at 1 year after surgery. There was no difference in the scores between the Excised Tendon (47 to 87) and Intact Tendon (53 to 89) groups. Conclusion: We concluded that the FDL muscle hypertrophies in response to a failing PT muscle. This hypertrophy continues after FDL transfer and medial displacement calcaneal osteotomy. With excision of the PT tendon, the FDL undergoes greater hypertrophy than if the tendon is left attached. The PT muscle continues to atrophy and undergoes complete fatty replacement if the tendon is excised. Transfer of the FDL and medial displacement calcaneal osteotomy produce a satisfactory improvement in hindfoot function; the outcome was the same whether the PT tendon was sacrificed or left intact.