Tibialis posterior muscle: the fifth compartment?

The posterior aspect of 51 embalmed cadaver legs in 50 cadavers was dissected to establish the prevalence of a separate compartment for the tibialis posterior muscle (TP). All dissections revealed the presence of a superficial and a deep posterior compartment. No distinct fascial septum separated the TP from the flexor digitorum longus (FDL) and flexor hallucis longus (FHL). We conclude that the TP does not commonly rest within its own osseofascial compartment and thus does not require isolated decompression for acute compartment syndrome of the leg. An incidental observation, frequently overlooked in the anatomy literature, was a supplemental tendon of origin of the FDL. In addition to the classically described tibial origin, several cadavers exhibited a proximal fibular tendon of origin for the FDL. One cadaver demonstrated the FDL to have an extensive fibular origin that completely covered the TP, forming a myotendinous fifth compartment. We feel that the variable fibular origin can explain the chronic exertional compartment syndrome of the TP described previously by Davey et al. and serves as a basis for a minor alteration in our fasciotomy technique.