Introduction: Total disc replacements have been speculated to rely on both successful surgical preparation and strong bony ongrowth for successful clinical outcomes. Current spinal arthroplasty designs utilize textured coatings, such as titanium plasma-spray, intended to increase the surface area resulting in long-lasting bone growth and robust mechanical fixation. However, the contribution of direct bony attachment to successful clinical fixation has not yet been established definitively. The goal of the present study was to document the extent of bony attachment to the surfaces of two clinically functional total disc replacements that were securely fixed at the time of revision.
Methods: Two compressible metal and polymeric disc replacements, one lumbar and one cervical, were evaluated for bony ongrowth following explantation. The cervical device was retrieved at 8 months due to a suspected immunological reaction, and the lumbar device at 28 months due to radiographic evidence of osteolysis. Non-destructive visual inspection was performed to determine possible failure mechanisms and grossly assess fixation. Devices were then embedded in polymethyl methacrylate (PMMA) and sliced using a diamond band saw into 5 coronal cuts for the lumbar specimen and 3 coronal cuts for the cervical specimen. High resolution photographs and contact micro-radiographs were taken to precisely assess bony ongrowth to the textured plasma-sprayed coating on the endplates.
Results: An initial inspection of both the lumbar and cervical devices, including the observation of large bone masses on the endplates, surgical extraction damage, and lack of radiographic migration suggested that both were well-fixed at the time of removal (Figure 1). After sectioning, micro-radiographs revealed radiolucent gaps between the endplates and bone mass on the lumbar device (Figure 2). Specifically, little direct contact between the bone and endplate surface was identified and the original surgical cuts were still visible. In contrast, direct bony attachment was visible on the posterior and middle sections of cervical endplate, but was absent from the anterior aspect.
Conclusions: The contribution of initial mechanical stability to the success of devices and long-term bony ongrowth has been well established in weight-bearing implant literature. Accordingly, total disc replacements have been suspected to rely heavily on initial bony contact for osseointegration. The results of the present report suggest both devices were clinically fixed at the time of removal and neither had complications associated with loosening. However, osseointegration due to new bony ongrowth was not observed in either device. Both devices in the present study have a titanium plasma-spray which has been shown to significantly enhance bony ongrowth and promote successful integration of new bone onto coated weight-bearing implants after as little as 6 months. However, the micro-radiographs of the lumbar endplate in the present analysis showed the precise straight-edged cuts formed by milling of the bone in surgical preparation and subsequent radiolucent gaps. The findings of the present study may suggest that different factors could influence overall clinical fixation such as the surgical preparation of the vertebral bone or the surface roughness of the treated endplates, and should be considered as a topic for future investigation.
