Background: The existence of a "fifth" compartment in the leg capable of developing distinct chronic exertional compartment syndrome remains a subject of controversy.
Hypothesis: Specific pressure recordings and dissection will confirm or disprove the existence of a fifth compartment.
Study design: Empirical anatomic study.
Methods: Radiopaque dye was injected directly into the tibialis posterior muscle of 25 embalmed cadaveric legs while intracompartmental pressure was monitored. Radiographs demonstrated dye distribution, and dissection-documented fascial and epimysial layers.
Results: Evidence was found that the fibular origin of the flexor digitorum longus muscle, when present, could create subcompartments within the deep posterior compartment. The nature of this attachment varied from being absent, to small (<8 cm), to extensive (>8 cm). The attachment partially covered the tibialis posterior muscle in the majority of the 14 legs that developed high pressures, and it was limited or absent in the 11 legs that did not. Radiographs demonstrated that the dye was confined to the tibialis posterior muscle in four legs.
Conclusions: No consistent fifth compartment exists in the leg; however, subcompartments within the deep posterior compartment created by the fibular origin of the flexor digitorum longus muscle may develop pressures congruent with chronic exertional compartment syndrome.
Clinical relevance: Potential deep posterior subcompartments demand accurate pressure investigation. A modified technique to decompress the entire deep posterior compartment, including the tibialis posterior muscle, is necessary for successful treatment of chronic exertional compartment syndrome.