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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: 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 1. Mean ± SD step-off height measurements (in mm) for the medial, lateral, superior, and inferior quadrants, and whole patella for matched (n = 20), unmatched (n = 20), and all (n = 40) patallas. Note that all patellas superior and lateral was statistically different than inferior (*0=0.01 +0=0.001) Circumferential Step-Off Height Matched Wiberg (n=20) Unmatched Wiberg (n=20) All Patellas (n=40) Medial 0.44 ± 0.21 0.57 ± 0.35 0.51 ± 0.30 Lateral 0.54 ± 0.20 0.66 ± 0.42 0.60 ± 0.33* Superior 0.57 ± 0.23 0.55 ± 0.21 0.56 ± 0.22+ Inferior 0.32 ± 0.17 0.41 ± 0.19 0.36 ± 0.18 Total 0.55 ± 0.18 0.60 ± 0.26 0.57 ± 0.22 Table 2. Mean ± SD surface deviation (in mm) for medial, lateral, superior, and inferior quadrants, and whole patella for matched (n =20) and unmatched (n = 20) Wiberg donor transplants Circumferential Step-Off Height Matched Wiberg (n=20) Unmatched Wiberg (n=20) P Medial 0.48 ± 0.20 0.51 ± 0.26 0.994 Lateral 0.42 ± 0.24 0.49 ± 0.22 0.889 Superior 0.48 ± 0.23 0.59 ± 0.30 0.576 Inferior 0.43 ± 0.24 0.58 ± 0.35 0.262 Total 0.50 ± 0.14 0.64 ± 0.25 0.049

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.

Background:Osteochondral allograft transplantation is an effective technique for repairing large lesions of the medial femoral condyle (MFC), but its use is limited by graft availability.Purpose/Hypothesis:The present study aimed to determine whether contralateral lateral femoral condyle (LFC) allografts can provide an acceptable surface match for posterolateral MFC lesions characteristic of classic osteochondritis dissecans (OCD). The hypothesis was that LFC and MFC allografts will provide similar surface contour matches in all 4 quadrants of the graft for posterolateral MFC lesions characteristic of OCD.Study Design:Controlled laboratory study.Methods:Ten fresh-frozen recipient human MFCs were each size-matched to 1 ipsilateral medial and 1 contralateral LFC donor (N = 30 condyles). After a nano–computed tomography (nano-CT) scan of the native recipient condyle, a 20-mm circular osteochondral “defect” was created 1 cm posterior and 1 cm medial to the roof of the intercondylar notch (n = 10). A size-matched, random-order donor MFC or LFC plug was then harvested, transplanted, and scanned with nano-CT. Nano-CT scans were then reconstructed, registered to the initial scan of the recipient MFC, and processed in MATLAB to determine the height deviation ( dRMS) between the native and donor surfaces and percentage area unacceptably (>1 mm) proud (% Aproud) and sunken (% Asunk). Circumferential step-off height ( hRMS) and percentage circumference unacceptably (>1 mm) proud (% Cproud) and sunken (% Csunk) were measured using DragonFly software. The process was then repeated for the other allograft plug.Results:Both MFC and LFC plugs showed acceptable step-off heights in all 4 quadrants (range, 0.53-0.94 mm). Neither allograft type nor location within the defect had a significant effect on step-off height ( hRMS), surface deviation ( dRMS), % Aproud, or % Asunk. In general, plugs were more unacceptably sunken than proud (MFC, 13.4% vs 2.4%; LFC, 13.2% vs 8.1%), although no significant differences in % Csunkwere seen between allograft types or locations within the defect. In LFC plugs, % Cproudin the lateral quadrant (28.0% ± 26.1%) was significantly greater compared with all other quadrants ( P = .0002).Conclusion:The present study demonstrates that 20-mm contralateral LFC allografts provide an acceptable surface match for posterolateral MFC lesions characteristic of OCD.Clinical Relevance:With comparable surface matching, MFC and LFC allografts can be expected to present similar stresses on the knee joint and achieve predictably positive clinical outcomes, thus improving donor availability and reducing surgical wait times for matches.

Disclosures: None Background: Current literature indicates that the risk of chondral and osteochondral injuries following patellofemoral instability events ranges from 40 to 96%. It is unclear based on current literature if there is an association between the number of instability events and the prevalence and severity of chondral and osteochondral injuries. Hypothesis/Purpose: The purpose of this study was to investigate the prevalence and severity of chondral and osteochondral injury following patellar dislocations based on number of dislocations and accounting for other patellofemoral instability risk factors (patella alta, trochlear dysplasia, growth status). Methods: Retrospective chart review was performed for patients under the age of 18 years with a prior history of traumatic patellar dislocation, documented number of instability events, and MRI available for review. Patients were grouped into those with 1 dislocation and >1 dislocation. A priori power analysis based on previous literature demonstrated a required minimum of 82 patients in each group (alpha=.05, power=90%, p1= 70%, p2=90%). Chart review was performed on all patients with diagnosis of patellofemoral instability for past 3 years to determine inclusion and record number of dislocations. MRI and radiographs were independently reviewed by a Musculoskeletal Radiologist and Orthopaedic Surgeon. Chondral injury severity (none, partial thickness, full thickness, osteochondral), location of injury, skeletal maturity, Caton-Deschamps index (CDI), and sulcus angle were recorded. Any discrepancies were settled by review by senior author. Poisson multivariate regression, chi-square, and descriptive statistics were used as appropriate. Results: 99 patients were included in each group. There was no statistically significant difference in the prevalence of chondral injury between patients with one dislocation versus more than one dislocation (64.65% versus 55.56%, p=.19) (Table 1). Multivariate regression analysis determined that no factor (patella alta, trochlear dysplasia, skeletal maturity, number of dislocations) was associated with prevalence of chondral injury. Multivariate regression analysis determined patella alta (CDI >1.3) was a significant factor in severity of chondral injury (p=.016). Conclusion: A history of multiple patellar dislocations is not associated with a higher prevalence or increased severity of chondral injury compared to a single patellar dislocation event. In patients with patella alta, a lower threshold for surgical management may be warranted due to its association with more severe cartilage injury, if not already present. [Table: see text]

Objectives: The purpose of this study was to determine whether contralateral lateral femoral condyle (LFC) allografts can provide an acceptable surface topography match for classic osteochondritis dessicans (OCD) lesions of the medial femoral condyle (MFC). Achievement of an acceptable donor-recipient articular surface match (1 mm deviation) has been associated with physiological joint stresses and predictably positive clinical outcomes. It was hypothesized that LFC and MFC allografts would show no differences in step-off height or surface deviation in all four quadrants of the graft. Methods: ample size calculation suggested ten groups of fresh frozen size-matched human condyles, each group consisting of a donor MFC, donor LFC, and recipient MFC. A 20 mm circular osteochondral “defect” simulating a “classic” OCD lesion was created in the recipient MFC. Its most anterior position was 1 cm posterior and 1 cm medial to the roof of the intercondylar notch. A randomly selected donor MFC or LFC plug was then harvested and transplanted using standard procedure (Fig 1A). The transplanted condyle was scanned with nano-CT, reconstructed (Fig 1B), registered to an initial scan of the recipient MFC, and processed with a custom MATLAB program to determine the surface root mean squared deviation (dRMS) between the native and donor surfaces (Fig 1C), percent area unacceptably proud (>1 mm; %Aproud) and sunken (1 mm; %Cproud) and sunken (< -1 mm; %Csunk) were measured (Fig 1D). The process was then repeated for the other allograft plug. Two-way mixed ANOVAs with Sidak corrections for multiple comparisons (α=0.05) were used. Exempt status was obtained from the University’s IRB. Results: Both MFC and LFC plugs showed acceptable step-off heights in all four quadrants. Neither allograft type nor location within the defect had a main effect on step-off height (hRMS). In general, plugs were more unacceptably sunken than proud, though no differences in %Csunk were seen between allograft types or locations within the defect. In LFC plugs, %Cproud was significantly greater laterally (by the intercondylar notch) compared to all other locations around the plug (p