Reverse Total Shoulder Arthroplasty with Proximal Bone Loss: A Biomechanical Comparison of Partially versus Fully Cemented Humeral Stems.

BACKGROUND: The appropriate amount of cementation at the time of reverse total shoulder arthroplasty with significant proximal bone loss or resection is unknown. Extensive cementation of a humeral prosthesis makes eventual revision arthroplasty more challenging, increasing the risk of periprosthetic fracture. We analyzed the degree of subsidence and torque tolerance of humeral components undergoing standard cementation technique versus our reduced polymethyl methacrylate (PMMA) protocol. Reduced cementation may provide sufficient biomechanical stability to resist physiologically relevant loads, while still permitting a clinically attainable torque for de-bonding the prosthesis.

METHODS: A total of 12 cadaveric humeri (6 matched pairs) underwent resection of 5 cm of bone distal to the greater tuberosity. Each pair of humeri underwent standard humeral arthroplasty preparation followed by either cementation using a 1.5 cm PMMA sphere at a location 3 cm inferior to the porous coating or standard full stem cementation. A 6-degrees-of-freedom (DOF) robot was used to perform all testing. Each humeral sample underwent 200 cycles of abduction, adduction, and forward elevation while being subjected to a physiologic compression force. Next, the samples were fixed in place and subjected to an increasing torque until implant-cement separation or failure occurred. Paired t-tests were used to compare mean implant subsidence versus a predetermined 5 mm threshold, as well as removal torque in matched samples.

RESULTS: Fully and partially cemented implants subsided 0.49mm (95% CI, 0.23 to 0.76 mm) and 1.85mm (95% CI, 0.41 to 3.29 mm), respectively, which were significantly less than the predetermined 5mm threshold (p<0.001 and p<0.01, respectively). Removal torque between fully cemented stems was 45.22 Nm (95% CI, 21.86 to 68.57 Nm), versus 9.26 Nm (95% CI, 2.59 to 15.93 Nm) for partially cemented samples (p=0.021). Every fully cemented humerus fractured during implant removal versus only one in the reduced cementation group. The mean donor age in our study was 76 years (range, 65 to 80 years). Only one matched pair of humeri belonged to a female donor with comorbid osteoporosis. The fractured humerus in the partially cemented group belonged to that donor.

CONCLUSION: Partially and fully cemented humeral prostheses had subsidence that was significantly less than 5 mm. Partially cemented stems required less removal torque for de-bonding of the component from the cement mantle. In all cases, removal of fully cemented stems resulted in humeral fracture. Reduced cementation of humeral prostheses may provide both sufficient biomechanical stability and ease of future component removal.