Our study utilized two ex-vivo methods of evaluating the integrity of the BFX cup – bone interface. Both cadaveric specimens and polyurethane foam blocks with similar mechanical properties to cancellous bone [12] were used. The resistance generated between the cup and foam block with edge loading was not affected by removal of up to 75% of the simulated DAR region. When tested under axial compression with the femoral prosthesis, all levels of DAR loss failed under loads that were in the supraphysiologic range (2510–4640 N) in both the cadaver specimens and foam blocks. These values were 12-23x larger than expected force during normal daily activity [4,13]. According to our ex-vivo data, it would appear that DAR loss levels typically observed in dogs with hip dysplasia do not significantly compromise BFX cup stability.
One major question to consider when testing press-fit cups is what failure mode is being represented. Our study did not attempt to test the effect of DAR loss on cup micromotion. Greater micromotion of the BFX cup may be present in acetabula with poor DAR, which could result in a fibrous interface and predispose to aseptic loosening, a complication seen in cemented canine THR [14]. Rather, both rim loading and centered loading tests simulated acute loosening accompanied by gross movement of the implant; thus our findings only give insight into initial cup stability. Further studies on canine acetabular cup micromotion are required to elucidate mechanical causes of aseptic loosening. Cyclic testing using a similar testing set-up may provide further insight into cup stability beyond the very early post-operative period.
While it would have been preferable to test varying degrees of DAR loss for the cadaveric component of the investigation, we elected to assess only one magnitude, 50% loss, for several reasons. The lack of significant difference between the 25, 50, and 75% loss in both edge and centered foam block loading suggested that the biomechanical consequences over a wide range DAR loss were roughly equivalent. Also, 50% DAR loss with our method of resection subjectively appeared to most closely replicate poor acetabular conformation in dogs with severe hip dysplasia. It is possible that removal of more than 50% DAR in the cadaveric specimens would have caused failure within a physiologic range of limb loading; however, a defect extending into the isciatic spine seemed to resemble far less common bone morphology, such as acetabular fracture malunion.
Robust stability despite partial loss of DAR can be explained by the design of BFX system, where the press-fit mechanism is thought to be predominately generated between the cranial and caudal acetabular margins. The most important factor in determining press fit cup stability in human THR is the ability of the cup to engage bone around the entire outer periphery [15]; however circumferential rim contact is not possible in dogs because the normal canine acetabulum is not hemispherical. Although we did not specifically test the contributions of the cranial or caudal acetabular margins to cup stability, our results suggest that the predominating press-fit mechanism of the BFX cup is achieved cranio-caudally rather than dorso-ventrally or circumferentially, as typically described for human THR systems.
Medialization of the cup by deeper reaming through the medial wall has been advocated for acetabula with poor DAR coverage, with the goal of increasing dorsal coverage of the acetabular cup [16]. Our results suggest that this approach may not be necessary, as we suspect the loads at failure were not compromised by loss of DAR to a clinically significant degree. In a study of medially displaced BFX cups in normal canine hemipelves [17], loads at failure were comparable to the loads observed in our cadaveric testing, both of which were well above the physiologic range. Based on these findings, we do not routinely advocate reaming through the medial wall as long as the cup is well engaged between the cranial and caudal acetabular margins, regardless of DAR bone stock.
There were several additional limitations to our study. Our DAR resection did not replicate microstructural changes that occur during remodeling with hip dysplasia; however, we suspect subchondral sclerosis observed with degenerative joint disease would have provided even more support than the normal porous cancellous bone. We were required to re-use cups throughout all testing. Debris was removed and no gross deformation of the cup or polyurethane lining was apparent, nor were any decreasing trends in total load to failure apparent in successive testing and reuse of the cups. Nevertheless, repeated cup use may have subtly damaged the porous surface and it would have been ideal to use new cups for each individual test. Cobalt-chromium bead-sintered BFX cups were used, and the results may not be directly applicable to the newer electron-beam-melted titanium BFX cups that are now commercially available. We attempted to standardize the force applied to the impactor during cup placement; however it was subjectively deemed that manual cup impaction as performed in a clinical setting was a more consistent method during pilot testing. Because we were unable to ensure identical impact strength for each specimen, it is possible that some inter-specimen variability was due to slightly inconsistent cup impaction. Mild variation in cup positioning may have also contributed to some variability. All cadaveric specimens failed by fracture of the acetabulum rather than cup dislodgement. It is conceivable that our testing set up was designed so that bone fracture was the only mechanism of failure, which very rarely occurs in clinical cases [18]; however, we utilized a set up for assessing canine THR cup biomechanics [17] where failure caused by cup dislodgement was observed. Finally, there are inherent limitations using displacement values that were required for estimating specimen stiffness from the materials testing machine, due to deformation of acetabular cup, femoral prosthesis, bone, and jig that were not individually quantified.
It is also prudent to note that while adequate cup stability may be attained in dogs with loss of DAR, other factors may contribute to cup loosening and dislodgement. In the clinical setting, intraoperative cup stability may be compromised by a number of technical errors, such as soft tissue entrapment, wobble during reaming causing inappropriate expansion of the acetabular bed, poor reaming alignment, and failure to deliver the cup coaxial to the reamed bed. To the authors’ knowledge, there is only one report of cup dislodgment with the BFX system [18]. In this case, inadequate cup stability was attributed to poor DAR bone stock and subsequent DAR fracture; however, post-operative radiographs suggested that reaming and cup placement was centered dorsally over the DAR itself. Thus in addition to lack of DAR, poor reaming alignment would have placed the cranial and caudal poles of the cup relatively dorsal to the cranial and caudal acetabular margins, which may not have generated appropriate craniocaudal press-fit. Our results may only apply to when there is precise execution of acetabular bed preparation and cup impaction.