Reverse total shoulder arthroplasty with proximal bone loss: a biomechanical comparison of partially vs. fully cemented humeral stems

Abstract

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 vs. 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 debonding 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-degree-of-freedom 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 vs. a predetermined 5-mm threshold, as well as removal torque in matched samples.

Results: Fully and partially cemented implants subsided 0.49 mm (95% CI 0.23-0.76 mm) and 1.85 mm (95% CI 0.41-3.29 mm), respectively, which were significantly less than the predetermined 5-mm threshold (P < .001 and P < .01, respectively). Removal torque between fully cemented stems was 45.22 Nm (95% CI 21.86-68.57 Nm), vs. 9.26 Nm (95% CI 2.59-15.93 Nm) for partially cemented samples (P = .021). Every fully cemented humerus fractured during implant removal vs. only 1 in the reduced-cementation group. The mean donor age in our study was 76 years (range, 65-80 years). Only 1 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 debonding 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.

Keywords: Reverse total shoulder arthroplasty; bone cement; humeral stems; implant subsidence; proximal humerus fracture; shoulder biomechanics.

Figure 1

(A) Initially, 5 cm of bone was resected distally from the humerus greater tuberosity. (B) Next, the resected humerus was potted in a PMMA cylinder in conjunction with the glenosphere component. PMMA, polymethyl methacrylate.

Figure 2

(A) Humeral stem with a 1.5-cm cement sphere evenly spread 3 cm from the porous coating. (B) Sphere measurement prior to application.

Figure 3

Robotic manipulator (KUKA) with the test humerus attached to the sensor and the glenosphere secured to the test base.

Figure 4

Torque tolerance testing setup with stationary vice capturing the humeral component. The humerus is potted in a PMMA cylinder, and the cylinder is attached to the KUKA robot, which applies progressive torque. PMMA, polymethyl methacrylate.

Figure 5

(A) Scatterplot of subsidence with 5-mm predetermined subsidence threshold. (B) Removal torque for each experimental cement condition. Bar graphs indicate mean subsidence and mean torque with respective 95% confidence intervals.