Summary

Anterior Slope-Modifying Osteotomy Alters Length Change Behaviour of the Superficial Medial Collateral Ligament

Abstract

Purpose/Hypothesis: In revision cases with high posterior tibial slope, combined slope-reducing osteotomy and anterior cruciate ligament reconstruction have shown promising results with reduced failure rates. However, potential disadvantages of this surgical procedure, like altering the length change pattern of the medial ligamentous structures and its implications, have not been discussed.
Thus, the aim of this study was to examine resulting length changes of the medial collateral ligament complex following an anterior slope-modifying osteotomy. It was hypothesized that (1) an slope decreasing (increasing) osteotomy shortens (lengthens) the fibers of the superficial medial collateral ligament (sMCL), while (2) the length change pattern of the posterior oblique ligament (POL) remain unaffected.

Methods

Eight cadaveric knee specimens underwent anatomical dissection to allow precise identification of 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 +10° to -15° in steps of five degrees. Threads were mounted between tibial and femoral pins positioned along the anterior, middle and posterior parts of the attachments of the sMCL and POL. The resulting length changes between the tibiofemoral pin combinations were recorded using a rotary encoder from 0° to 120° knee flexion. The level of isometry was determined by the total strain range (TSR), which was calculated by subtracting the minimum strain from the maximum strain. A one-way repeated-measures ANOVA was performed for each structure to compare the length change and TSR across flexion angles and degrees of slope modification with the initial measure at 0° knee flexion. Statistical significance was set to p<0.05.

Results

All fiber region of the sMCL shortened gradually with slope reduction (p <.001) and lengthened with slope increase (p <.001), corresponding to the size of the osteotomy gap. This resulted in a negative/positive parallel shift of the length change curve, whereby the anterior fibers of the sMCL (mean decrease/increase in length of -6.85  3.01 % and 5.92  2.34 %) were more affected than the posterior fibers (mean decrease/increase of length -3.64  2.32 % and 1.59  0.98 %, p <.01). Regardless the fiber region, the POL were not significantly affected by an anterior slope modification (n.s.) and displayed a mean shortening of 42 % towards knee flexion.

Conclusion

Anterior slope-modifying osteotomies caused reciprocal length changes of the sMCL, which predominantly effected its anterior fibers. An anterior opening wedge osteotomy increased and an anterior closing wedge osteotomy decreased the lengths of the sMCL, while the length change pattern of the POL remained unaffected. Surgeons should be aware of these length changes and may perform a reefing or partial release of the sMCL in case of a subsequent medial instability or over-constraining.