Abstract
Introduction This is the largest analysis to date of any retrieved porous ingrowth disk replacement prostheses. In distinction to prior reports of retrieved implants which were conducted like “airplane crash” type pseudoanalyses, in this series the position of the components was known in vivo prior to implant removal. The digitized radiographs were used to determine if the components were in ideal, suboptimal, or poor position. There were thirty cervical disk replacements and thirty-eight lumbar disk replacements which comprised the basis of this analysis.
Methods Quantitative histomorphometry, microradiography, and histology were performed on all 68 vertebral endplates. Scanning electron microscopy was performed on ten. All 24 caprine model, 34 non-human primates, and 10 human explants with titanium calcium phosphate porous ingrowth surface were manufactured by the same vendor, D.O.T., which provides the same porous ingrowth coating for several FDA approved total hip replacements. Group I – Ideal placement, was defined as Charité or PCM Artificial Disc replacement within 3 mm of exact central axis in both the coronal planes and mid-sagittal planes (2 mm posterior to the midpoint of the vertebral body in the sagittal plane for Charité only).The endplates of the prosthesis also had to be within 5 degrees of angulation of the bony end-plate or within 5 degrees of angulation of the perpendicular axis of the vertebral body. Group II – Suboptimal placement, was defined as Charité or PCM Artificial Disc placement from 3 mm to 5 mm from exact central placement in at least one axis In addition the prosthetic endplate had to be from 5 degrees to 10 degrees of perpendicular vertebral body orientation. Group III – Poor placement, was defined as greater than 5 mm from exact central placement in at least one axis or the endplate was greater than 10 degrees off angle. Three separate observers judged the measurements of axes and made a determination of prosthesis placement after correction for magnification error.
Results The mean length of time in biologic conditions to monitor reabsorption and incorporation of the ingrowth surface was a mean of 10.5 months (range 6 to 33 months). This is the first study finding a correlation between the position of the components and amount of successful bony ingrowth. A representative group was: Ideal 50.9 +/− 13 % ingrowth, Suboptimal placement, 49.3 +/−18 % ingrowth, and Poor, 33.0 +/− 29.2 % ingrowth. There was trend but not statistically significant (F= 1.78, p = .186). The mean ingrowth of prostheses in poor and suboptimal position (defined by axis off by 3mm in either AP or Lateral plane) was 43.2 %. Whereas the mean ingrowth of prostheses inserted in “ideal position” within 3 mm of the optimal prosthesis axis in both planes was 46.4 %. The definition of successful biologic ingrowth in the extremities for total joint replacement is porous ingrowth over 30 %, which was achieved in 58 / 68 (85.3 %) of vertebral endplates.
Discussion The porous ingrowth TiCaP bioactive technology permits osseointegration despite non-ideal positioning. The surgeon’s technical shortcomings to place the prosthesis in ideal position were more than compensated for as 85.3 % of the components were successfully ingrown and biologically fixed to the vertebral trabeculae at the time of explantation. There were no cases of osteolysis or biomaterial failure encountered in this retrieval study.
The abstracts were prepared by Assoc Prof Bruce McPhee. Correspondence should be addressed to him at the Division of Orthopaedics, The University of Queensland, Clinical Sciences Building, Royal Brisbane Hospital, Herston, Brisbane, 4029, Australia.
