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Arthroscopic assessment of the posterior bundle of the medial collateral ligament's (pMCL) role in elbow stability
Michael J Niemann, MD
1, Lexi Bosetti, BS
2, Jefferson Li, MD
2, John Ibrahim, MD
2, Michael Hausman, MD
3; Mark E. Baratz, MD
4(1)West Virginia University, Morgantown, WV, (2)UPMC, Pittsburgh, PA, (3)Mount Sinai, New York, NY, (4)Hand and Upper Extremity Surgery, University of Pittsburgh Medical Center, Washington, PA
Introduction Recent literature suggests that the posterior bundle of the medial ulnar collateral ligament (pMCL), plays an important and underappreciated role in elbow stability. Golan et al demonstrated that Posteromedial rotatory instability (PMRI) of the elbow can occur in the setting of isolated transection of the pMCL without involvement of the coronoid or anterior bundle of the MCL (aMCL). When the pMCL is incompetent, the forearm rotates relative to the humerus as valgus and pronation stress is applied. With rotation gapping occurs at the posteromedial ulnohumeral joint, with the aMCL acting as the pivot point. Arthroscopy can be a useful clinical tool for assessing subtle elbow instability, but methods for assessing PMRI associated with disruption of the pMCL have not previously been described.
Materials and Methods Nine fresh-frozen unpaired cadaver arms free of pre-existing elbow instability, contracture, or prior surgery were evaluated arthroscopically. The pMCL was identified and marked in thirds for progressive sectioning with preservation of the aMCL (Figure 1A). Gapping of the posteromedial joint surface was measured from the posterior compartment using brass rods of known diameters (Figure 1B). Rotation of the forearm relative to the humerus was measured from the posterior compartment by assessing translation of a notch in the olecranon tip relative to a drill hole of a known size in the distal humerus when the elbow was stressed in pronation and valgus (Figure 1C). Rotation of the forearm relative to the humerus was also measured from the posterolateral compartment by assessing change in coverage of the ulnar surface with pronation and valgus stress at the proximal radioulnar joint (Figure 1D). Posteromedial gapping and rotation (from both the posterolateral and posterior compartments) was assessed with the pMCL intact, after partial and complete sectioning of the pMCL.
Results and Discussion The mean gapping of the posteromedial joint was 1.2mm with the pMCL intact and 5.1mm after complete sectioning of the pMCL. The average rotation measured from the posterior compartment was 0.6mm with the pMCL intact and 3.6mm after complete sectioning of the pMCL. The average rotation measured from the posterolateral compartment was 0.7mm with the pMCL intact and 3.8mm after complete sectioning of the pMCL. Progressive posteromedial gapping and rotatory instability was seen with progressive sectioning of the pMCL. This supports the importance of the pMCL in elbow instability and demonstrates an arthroscopic technique for assessing PMRI.
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