Salka, Nabeel

Background: Cartilage transplants in the patellofemoral joint have demonstrated lower success rates than in the femoral condyles. This is likely related to the more complex morphology and biomechanics of the joint. While previous studies have evaluated cartilage surface matching and congruence, little work has been done to study the associated subchondral bone congruency in these grafts. Purpose: To determine if differences in patellar morphology play a role in the alignment of the donor patellar osteochondral allograft subchondral bone with the native patellar subchondral bone. Study Design: Controlled laboratory study. Methods: A total of 20 (10 Wiberg I and 10 Wiberg II/III) fresh-frozen human patellae were designated as recipients and size-matched to both a Wiberg I and a Wiberg II/III patellar donor. A 16-mm osteochondral allograft transplant to the central ridge of the patella was performed in random order with each matched donor. Transplanted patellae underwent a nano—computed tomography (nano-CT) scan, were digitally reconstructed, and were superimposed on the initial nano-CT scan of the native recipient patella. MATLAB was used to determine the surface height deviation between the native and donor subchondral bone surfaces. DragonFly 3-dimensional imaging software was used to measure subchondral bone step-off heights at the native-donor interface. Differences between matched and unmatched grafts were compared using a 2-way analysis of variance and the Sidak post hoc test. Results: Subchondral bone surface deviation did not differ between Wiberg matched and unmatched allografts. The step-off height was significantly greater in unmatched (1.38 ± 0.49 mm) compared with matched (1.14 ± 0.52 mm) plugs ( P = .015). The lateral quadrant step-off differed between matched (0.89 ± 0.43 mm) and unmatched (1.60 ± 0.78 mm) grafts ( P = .007). Conclusion: While unmatched Wiberg patellar osteochondral allograft implantation did not result in significantly different subchondral bone surface height deviations, there was a significant difference in the circumferential subchondral bone step-off height in the lateral quadrant. Further investigation using finite element analysis modeling will help determine the role of subchondral bone surface on shear and compression force distributions in these areas. Clinical Relevance: Given that subchondral bone stiffness and morphology play a role in cartilage health, subchondral bone congruency may play a role in graft survival. Understanding how this congruency plays a role in cartilage force distribution will help surgeons improve the long-term success of osteochondral allograft transplants.

Objectives: Recent research has shown that implanting a patellar osteochondral allograft with a non-matched surface morphology (i.e., Wiberg classification) does not create increased chondral surface deviation or circumferential step-off in the donor plug compared to the native patella. While much of the research on patellar osteochondral allografts has been focused on chondral surface matching, little has been done to determine if the subchondral bone alignment at the donor:native interface plays a role in graft healing, local force distribution, and long term success of the allograft transplant. Previous work in our lab demonstrated that even when the patellar cartilage surface was well matched, notable differences in subchondral bone alignment were observed. The purpose of this study was therefore to use surface contour mapping of subchondral bone to determine if differences in Wiberg classification play a role in the ability of donor patellar osteochondral allograft subchondral bone to align with the native patellar subchondral bone when treating osteochondral defects of the patellar apex. The hypothesis was that patellar surface morphology would have an effect on subchondral bone surface height deviation and circumferential step-off when performing osteochondral allograft transplants of the patellar apex. Methods: Sixty fresh frozen human patellae were acquired from a national donor procurement company. Twenty (10 Wiberg I and 10 Wiberg II/III) patellae were designated as the recipient and then nano-CT scanned. Each recipient was size-matched (within ±2mm tibial width) to both a Wiberg I and a Wiberg II/III patellar donor. A 16mm circular osteochondral “defect” centered on the central ridge of the patella was then created in the recipient patella. A randomly-ordered donor Wiberg I or Wiberg II/III plug was harvested from a homologous location and transplanted into the recipient. The recipient was then nano-CT scanner, digitally reconstructed, and superimposed on the initial nano-CT scan of the native recipient patella. After careful atraumatic removal of the first donor plug, the process was repeated using the other allograft plug. MATLAB was used to determine the root mean square (RMS) surface height deviation between the native and donor subchondral bone surfaces. Dragonfly 3D imaging software was used to measure the RMS subchondral bone step-off height at 3° increments around the circumference of the graft. Surface height deviation and circumferential step-off height were analyzed for the whole surface and by quadrant to determine if there were local differences. ANOVA was used to compare surface deviation and step-off heights between matched and unmatched grafts. Sidak’s multiple comparison test was used to complete sub-analysis between patellar graft quadrants. Comparisons were made between matched and unmatched grafts in terms of the RMS surface height deviation and step-off, as well as in the percentage of measurements that were more than 0.5mm, 1mm, and 2mm proud or sunken relative to the native surface. Results: There were no significant differences in RMS subchondral bone surface height deviation between matched and unmatched Wiberg plugs as a whole or by quadrant (RMS range = 0.69 to 0.97mm, p = 0.45 – 1.0). There was a significant difference in RMS circumferential step-off height between matched (1.14 ± 0.52mm) and unmatched (1.38 ± 0.49mm) Wiberg plugs ( p=0.015). The majority of these increased step-off measurements occurred in the lateral quadrant with lateral quadrant RMS step-off of 0.89 ± 0.43mm in matched grafts and 1.60 ± 0.78mm in unmatched grafts ( p=0.007). There was also a significant difference in the percent of step-off measurements greater than 2mm sunken in the lateral quadrant between matched and unmatched grafts (5.17 ± 20.87% matched, 24.5 ± 36.39% unmatched, p=0.028). There were no significant differences between matched and unmatched grafts for any other comparison using 0.5, 1, or 2mm cut-offs for circumferential step-off or surface height deviation. Combining all allografts, the respective proportion of surface deviation and circumferential step-off height measurements that were above the stated thresholds were as follows: 31% and 34% for a 0.5mm threshold, 15% and 21% for a 1mm threshold, and 2% and 8% for a 2mm threshold. Conclusions: While unmatched Wiberg patella osteochondral allograft implantation did not result in significantly different subchondral bone surface height deviations, there were significant differences in circumferential subchondral bone step-off heights. The majority of step-off height differences between Wiberg matched and unmatched osteochondral allografts occurred in the lateral quadrant. In comparison to previous data evaluating differences in the cartilage surface match in these patellar OCA transplants, the deviations and step-off heights in the subchondral bone identified in the current study were approximately 0.5mm greater than the differences in the cartilage surface. These findings therefore suggest there is greater variability in the alignment of the subchondral bone in these patellar osteochondral allografts than there is in the cartilage surface. Further investigation using finite element analysis modeling will help determine the implications of subchondral bone surface deviation and circumferential step-off on local cartilage:bone compression and shear force distribution. These studies may shed light on the mechanisms of failure in patellar osteochondral transplants and may help to better understand the contribution of subchondral bone alignment in OCA healing and long-term outcome.

Objective The purpose of this study was to determine if differences in Wiberg classification play a role in the ability of donor patellar osteochondral allografts to match the native patellar surface when treating osteochondral defects of the patellar apex. Design Twenty (10 Wiberg I and 10 Wiberg II/III) human patellae were designated as the recipient. Each recipient was size-matched to both a Wiberg I and a Wiberg II/III patellar donor. A 16-mm circular osteochondral “defect” was created on the central ridge of the recipient patella. The randomly ordered donor Wiberg I or Wiberg II/III plug was harvested from a homologous location and transplanted into the recipient. The recipient was then nano-CT (computed tomography) scanned, digitally reconstructed, registered to the initial nano-CT scan of the recipient patella, and processed to determine root mean squared circumferential step-off heights as well as surface height deviation. The process was then repeated for the other allograft plug. Results There was no significant difference in mean step-off height between matched and unmatched Wiberg plugs; however, there was a statistically significant difference in surface height deviation over the whole surface (0.50 mm and 0.64 mm respectively, P = 0.03). This difference of 0.14 mm is not felt to be clinically significant. Tibial width was correlated to patellar width ( r = 0.82) and patellar height ( r = 0.68). Conclusions For osteochondral allograft sizes up to 16 mm there appears to be no advantage to match donor and recipient patellar morphology. Further study is warranted to evaluate defects requiring larger graft sizes.

Objectives: Osteochondral allografts of the patellar are currently matched solely based on tibial width. It is currently unknown whether matching by tibial width is a reasonable surrogate measurement to allow for optimum chondral surface matching or if patellar size and/or surface morphology (i.e., Wiberg classification) should be taken into account. This consideration may be especially important for chondral defects on the patellar apex. The purpose of this study was to use circumferential step-off height and chondral surface mapping to determine if differences in patellar surface morphology (i.e., Wiberg classification) play a role in the ability of donor patellar osteochondral allografts to match the native patellar surface when treating osteochondral defects involving the central ridge of the patella. The secondary purpose was to explore the relationship between tibial width and patellar size (width and height) to determine if tibial width strongly related to patellar size to allow it to act as a surrogate measure for patellar size. Methods: Twenty (10 Wiberg I and 10 Wiberg II/III) fresh frozen patellae were designated as the recipient. Each recipient was size-matched (within ± 2mm tibial width) to both a Wiberg I and a Wiberg II/III patellar donor to produce 20 size-matched trios. All patellas were classified as Wiberg I, II, or III by visual inspection. The patellar height and widths were also measured. The recipient patella underwent initial nanoCT scanning to quantify the native chondral surface morphology. A 16mm circular osteochondral “defect” centered on the central ridge of the patella was then created in the recipient patella. Within each set of three patellae, the donor Wiberg I and Wiberg II/III patellae were randomly assigned, using a random number generator, to be transplanted first or second. The randomly ordered donor Wiberg I or Wiberg II/III plug was harvested from a homologous location and transplanted into the recipient. The recipient was then nano-CT scanned, digitally reconstructed, registered to the initial nano-CT scan of the recipient patella. It was then processed in Dragon Fly to determine circumferential step-off heights between the native and donor surfaces at three degree intervals. This was calculated for the entire circumference and for each quadrant (superior, medial, inferior, lateral) to determine if the ste-off heights varied by locations. MATLAB was used to determine the height deviation (dRMS) between the native and donor surfaces at over 3000 surface points (Figure 1). The initial transplant was carefully removed and the process was then repeated for the other donor allograft. Pearson correlation coefficient, 2-way ANOVA with Tukey’s multiple comparison, and paired t-tests were used when appropriate. Sample size of 10 trios was determined based on previous work in our lab (clinically relevant difference of 0.75mm, SD = 0.5mm, α = 0.05, power 0.8; 7 samples per group). Results: There was no significant difference in mean step-off heights between matched and unmatched Wiberg allograft plugs (Table 1). When analyzing all patellas, the superior (p = 0.01) and lateral (p = 0.001) quadrants demonstrated step-off heights that were significantly greater compared to the inferior quadrant, however these findings were not clinically significant. There was a statistically significant difference in height deviation over the whole surface between native and donor plugs when comparing matched and unmatched Wiberg plugs (p=0.049), however this finding was not clinically significant (Table 2). There was no difference across individual quadrants. There was a linear correlation when comparing tibial width to patellar width (r = 0.82) and patellar height (r = 0.68). Conclusions: Differences in Wiberg classification did not lead to clinically relevant differences in step-off height or surface height deviations for the whole donor plug or by quadrant. Tibial width is a reasonable measure to predict patellar size (width and height). It is therefore reasonable to continue matching osteochondral allografts of the patella based on the easy to measure value of tibial width without consideration for patellar size or Wiberg classification. [Table: see text][Table: see text]