Summary
Anterior closed wedge osteotomies decrease the strain of the MCL, while anterior open wedge osteotomies increase the strain of the MCL.
Abstract
Background
A slope decreasing osteotomy has been suggested to reduce the graft rupture rate in anterior cruciate ligament revision cases with high posterior tibial slope. However, negative effects of this surgical procedure, such as changing the length change pattern of the medial ligamentous structures, have not yet been discussed.
Purpose
The aim of this study was to examine the length changes of the medial ligamentous structures following an anterior slope-modifying osteotomy. It was hypothesized that (1) a slope decreasing (increasing) osteotomy slackens (tightens) the fibers of the superficial medial collateral ligament (sMCL), while (2) the fibers of the posterior oblique ligament (POL) remain unaffected.
Methods
Eight cadaveric knee specimens underwent anatomical dissection to precisely identify the medial ligamentous structures. The knees were mounted in a custom-made kinematics rig with loaded quadriceps muscle and iliotibial tract using cables and hanging weights. An anterior slope-modifying osteotomy was performed and fixed using an external fixator, which allowed a gradual slope modification from -15 to +10 mm wedge height in five mm increments. Threads were mounted between tibial and femoral pins positioned in the anterior, middle and posterior parts of the attachments of the sMCL and POL. For each level of slope modification, the resulting length changes between the tibiofemoral pin combinations were recorded using a rotary encoder from 0° to 120° knee flexion.
Results
A slope modifying osteotomy had a significant effect (p < 0.001) on the length change pattern of the anterior and middle fibers of the sMCL. This resulted in a progressive increase in strain for the anterior (5.9 ± 2.3 % - 3.3 ± 2.1 %) and middle (1.9 ± 1 % - 3.5 ± 1.3 %) sMCL fibres for a 5 – 10 mm slope increasing osteotomy from 0 – 120°. Conversely, a 5 – 15 mm slope decreasing (anterior closed wedge) osteotomy resulted in a decrease in strain for the anterior (1.5 ± 1.5 % - 6.9 ± 3 %) and middle (0.8 ± 0.7 % - 3.9 ± 0.9 %) sMCL fibres from 0-120° knee flexion. The length change pattern of the posterior fibres and the POL did not change significantly (n.s.). Regardless of slope modification, the overall length change pattern of all tibiofemoral combination were maintained.
Conclusion
Slope modifying osteotomies altered the length change pattern of the medial collateral ligament, so that an anterior opening osteotomy increased and an anterior closing osteotomy decreased the strain in the anterior and middle fibers of the superficial medial collateral ligament. Similar to medial open and closed wedge osteotomies surgeons should be aware of this and may perform a reefing or partial release of these structures.