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Shear resistance of a posterior lumbar total joint replacement prosthesis
Avinash G Patwardhan1, Robert M Havey2, Scott D Hodges3
1Musculoskeletal Biomechanics Laboratory, Edward Hines Jr. VA Hospital, Hines, IL, USA; Department of Orthopaedic Surgery and Rehabilitation, Loyola University Chicago, Maywood, IL, USA.
Background Context:
Anterior total disc replacement in the lumbar spine does not address facet pathology or central/recess stenosis. A novel posterior-based motion-preserving lumbar total joint replacement (Lumbar TJR) allows direct neural decompression with laminectomy and bilateral facet resection, along with complete discectomy. The destabilization caused by a wide decompression raises the question: How stable is the lumbar TJR implant under expected physiological (combined compression and shear) forces?
Purpose:
To investigate the shear resistance of L5-S1 reconstructed with bilateral posterior lumbar TJR prostheses, implanted after bilateral facetectomy and discectomy.
Study Design:
A biomechanical study using human lumbosacral spine specimens.
Methods:
Seven (7) L1-S1 human cadaveric spine specimens were tested, each in the following sequence: (A) intact, (B) after L5-S1 bilateral facetectomy and discectomy, and (C) after lumbar TJR prostheses were implanted bilaterally at L5-S1. Stability was quantified by measuring anterior slip of L5 and angular displacement at L5-S1 under a net force of 700N applied across L5-S1, first with minimal shear (S∼0N, C=700N), then with shear-to-compression (S:C) ratios of 1:1 (S=495N, C=495N) and 1.7:1 (S=600N, C=350N), simulating increasing shear-loading scenarios.
Financial Disclosures:
Five of the 8 authors own shares in the company that manufactures the TJR implant. No other conflicts are noted. Supported in part by research funds received from the sponsor, which were paid directly to the institution to offset expenses related to technician salaries and materials. The Lumbar TJR device is investigational.
Results:
In the intact L5-S1 segment, statistically significant anterior slip was created under load with S:C ratios of 1:1 (0.76±0.52 mm) and 1.7:1 (0.83±0.51 mm) when compared to loading with minimal shear (0.03±0.17 mm) (p<.05). After bilateral facetectomy and discectomy, the L5 slip under load increased to 0.29±0.63 mm, 3.2±1.4 mm (p<.05), and 3.6±1.4 mm (p<.05) at S:C ratios of 0:1, 1:1, and 1.7:1, respectively. The lumbar TJR significantly reduced the anterior slip of L5 to 1.4±0.6 mm and 1.6±0.6 mm at S:C ratios of 1:1 and 1.7:1, respectively (p<.05). The lumbar TJR limited the L5-S1 anterior slip to the intact level (p=.18). Likewise, the lumbar TJR restored the L5-S1 angular motion to the intact level (p=. 96) under combined shear and compression loading.
Conclusions:
The lumbar TJR prosthesis restored the anterior slip and angular displacement at L5-S1 to the levels of an intact segment. These biomechanical results show the lumbar TJR resists anterior shear forces which are of primary concern after wide posterior decompression involving laminectomy, bilateral facetectomy and discectomy.
Clinical Significance:
The ability of the reconstructed L5-S1 segment to resist anterior listhesis is a primary concern in early postoperative period after wide posterior decompression involving laminectomy, bilateral facetectomy and discectomy. This biomechanical study provides evidence of the early postoperative stability of the lumbar TJR. Further studies, including in-vivo clinical studies, will be important to corroborate these findings.
