Anatomy of the distal knee joint and pyarthrosis following external fixation.

OBJECTIVE: To determine the limits of the distal synovial reflection of the human knee joint. SPECIMENS: Six paired knees studied by magnetic resonance imaging (MRI), fluoroscopic arthrography, and gross dissection. The right knees of five patients with chronic idiopathic knee effusions were studied by MRI. Cadaveric knees were injected with saline prior to MRI. The joint capsules were dissected to visualize local anatomy and check for capsular tears. In each modality (MRI, fluoroscopy, and dissection), the most distal extent of knee synovial fluid was measured.
RESULTS: The right versus left agreement for paired specimens was generally two to three millimeters. Some specimens showed asymmetric capsular reflection. Medial fluid was identified at distances greater than forty-nine millimeters from the subchondral bone in seven knees and less than fifteen millimeters in four knees (range 0 to 70 millimeters, mean thirty-three millimeters). Laterally, the range was ten to thirty-five millimeters (mean twenty-three millimeters). In six of the twelve cadaveric specimens, there was evidence of a communication between the knee joint and the proximal tibiofibularjoint. In the knees of volunteers, joint fluid tracked medially to a range of ten to fifty millimeters and laterally to a range of six to fifteen millimeters, with means of twenty-six and eleven millimeters, respectively. The knees of the volunteers had no evidence of tibiofibular joint communication with the knee.
CONCLUSION: Insertion of external fixation pins within sixty to seventy millimeters of the proximal articular surface of the tibia is associated with a high probability of synovial penetration and possibly provides a conduit for the introduction of bacteria, which may be etiologic in iatrogenic pyarthrosis.