Study: Knee Stability Tied to Medial Line
A group of Belgian researchers has found that the level of the medial joint line is the key prerequisite for normal joint stability. Their study, “Raising the Joint Line in TKA is Associated With Mid-flexion Laxity: A Study in Cadaver Knees,” appears in the March 2018 edition of Clinical Orthopaedics and Related Research.
Thomas Luyckx, M.D., Ph.D., with the Department of Orthopaedic Surgery at University Hospitals Leuven in Belgium, co-author on the study told OTW, “We have the experience that instability after primary TKA [total knee arthroplasty] is an important clinical problem. In fact, instability has become the number one reason for early revisions.”
“It can be considered a technical problem and as such one that can be avoided. We wanted to add to the understanding of the causes of this instability and how to try to avoid them.”
“In this cadaveric study, a TKA prosthesis was implanted in 10 fresh frozen nonarthritic cadaveric knees with restoration of the medial joint line at its original level (TKA0). Coronal plane stability was measured at 0°, 30°, 60°, 90°, and 120° flexion using a navigation system while applying an instrumented 9.8 Nm varus and valgus force moment. The joint line then was raised in two steps by re-cutting the distal and posterior femur by an extra 2 mm (TKA2) and 4 mm (TKA4), downsizing the femoral component and, respectively, adding a 2- and a 4-mm thicker insert.”
“When the joint line is raised, a significant mid-flexion laxity was observed. This effect was quite strong. A 50% increase in laxity with 2mm raised joint line and 100% with 4mm raise. This instability occurred even though the flexion and extension gap remained equal and balanced.”
“If you want to keep the joint line at its original level in an osteoarthritic knee, you have to consider cartilage wear. Typically, there is loss of 2mm of distal medial femoral cartilage. If you don’t compensate for this, you automatically raise the joint line. You also create a mismatch of the extension gap with the flexion gap because in most cases the posterior medial cartilage is still present.”
“The most important finding is that if you want to maintain the normal coronal plane stability of the knee, you have to keep the medial joint line at its original level, both in extension and flexion. The level of the medial joint line seems to be the essential pre-requisite for normal joint stability.”
