Pearls and Pitfalls in Difficult Patellar Osteochondral Allograft Transplantation

Abstract

Background:

Osteochondral allograft (OCA) transplantation is a durable option for patients with symptomatic chondral or osteochondral defects involving the patella; however, the unique shape and contour of the patella can make successful OCA transplantation challenging.

Indications:

Patellar OCA transplantations are indicated in the setting of a symptomatic patellar chondral or osteochondral lesion that has failed conservative management. A concomitant offloading tibial tubercle osteotomy (TTO) may be added depending on the location of the lesion and the posteriorization of the tibial tubercle relative to the trochlea.

Technique Description:

Donor size and shape-matched patellae should be obtained before the procedure. The patella is exposed via a laterally based arthrotomy, through which a lateral retinacular lengthening is performed. When TTO is indicated, the osteotomy is performed first to facilitate patellar eversion and exposure. The recipient defect is sized and matched to a donor patellar cartilage surface. Using a guide pin, the defect is then scored and reamed using constant cold flow irrigation. The donor cartilage plug is then prepared via cold irrigation reaming, cutting to the proper depth, and lavaging with saline and carbon dioxide gas to remove bone marrow elements and antigenicity. It is then press-fit into the recipient defect location.

Results:

We represent pearls and pitfalls at all stages during patellar OCA transplantation in the setting of large focal defects in various locations. There has been excellent survivorship of OCA plugs in the patellofemoral joint up to 8 to 10 years, with a significant decrease in survivorship with the use of shell-type OCAs.

Discussion/Conclusion:

Patellar OCA transplantation is a complex procedure that requires further consideration in cases of incongruent topography of the cartilage surface. Ensuring adequate matching between donor and recipient grafts and implementing steps to optimize integration and graft biology can help achieve optimal outcomes.

Patient Consent Disclosure Statement:

The author(s) attests that consent has been obtained from any patient(s) appearing in this publication. If the individual may be identifiable, the author(s) has included a statement of release or other written form of approval from the patient(s) with this submission for publication.

Graphical Abstract

This is a visual representation of the abstract.

Video Transcript

The following video highlights pearls and pitfalls in the challenging procedure of patellar osteochondral allograft (OCA) transplantation, focusing on the surgical aspects rather than indications or postoperative care. There are over 900,000 articular cartilage injuries diagnosed annually, with over 200,000 cartilage surgeries performed. Up to 52% of patients with knee pain are diagnosed with cartilage lesions of the patellofemoral joint, and a variety of cartilage treatment methods are available. ⁶

Background

While there is no evidence-based consensus on the treatment of patellofemoral chondral lesions, OCAs can be used for revision cases, bipolar lesions, uncontained defects, and lesions with subchondral bony involvement. Surface-based treatments are reserved for surface-based chondral lesions, contained lesions, and those without subchondral bone edema, cysts, or plate irregularities. Treatment of the patellofemoral joint can be combined with soft tissue balancing, such as lateral retinacular releases or lengthening, medial patellofemoral ligament (MPFL) reconstructions, and offloading tibial tubercle osteotomies (TTOs). While all are viable treatment options for chondral lesions in the patellofemoral joint, there are cons to each, with OCAs being technically difficult, especially regarding donor-recipient contour matching, with the possibility of catastrophic failure. Matrix-associated autologous chondrocyte implantation (MACI) or autologous chondrocyte implantation (ACI) treatment options are expensive and require a 2-stage procedure, involving initial biopsy and second-stage implantation. TTO, in particular, is at risk for nonunion and peri-implant fracture, and requires a nonweightbearing period.

Indications

Successful patellar OCA transplantation begins with ordering a shape- and size-matched donor patella. Factors to consider include patellar shape, such as varying degrees of dysplasia according to the Wiberg classification, and, more importantly, overall patellar width and cartilage length. It is essential to match these 2 parameters, even more so than the percentage of lateral facet in dysplastic patellae, and all factors of size and shape take precedence over age and sex of the donor grafts, as even though the literature has demonstrated improved outcomes with matched grafts, to practically obtain a size-, shape-, age-, and sex-matched patella would limit the available grafts severely.

The next step is exposure. For the vast majority of patellar defects, a lateral parapatellar arthrotomy can be used, with a lateral lengthening performed if patellar tilt or instability coexists. This preserves the medial vascular supply and also, in cases of concomitant patellar instability, leaves the medial layers intact for MPFL reconstruction. A centimeter or 2 of vastus lateralis can be cut proximally to help with exposure, but care must be taken during closure not to inadvertently shift the lateral structures. Incising across the infrapatellar fat pad, posterior to the patellar tendon, can also significantly aid in patellar eversion for exposure. Depending on the location of the lesion and the relative posteriorization of the tubercle to the trochlea, or in other words, a more negative sagittal tibial tubercle-trochlear groove distance, an anteriorizing TTO can be performed. Either a 60° or 90° cut can be made, depending on desired anteriorization, and this TTO can be performed in the same or a separate incision. The osteotomy should be performed before the OCA to aid in patellar exposure, with fixation performed after OCA transplantation.

TECHNIQUE Description

When prepping the recipient site, the appropriate diameter size should be chosen to encompass the lesion. The donor patella should be held next to the defect to plan the correct harvest site. A marking pen outline on both the recipient and donor can aid in a congruent OCA. For medial and lateral defects, the primary planning step is to ensure that both the recipient and donor are on the same side of the central ridge. For central defects encompassing the ridge, this presents a unique challenge, and planning requires a bit more patience. With the marking pen technique, the surgeon is able to see that the donor and recipient will share an excellent topographic match on the medial side, but laterally, there are 2 areas on the donor patella that sit more recessed than on the recipient. The surgeon can then adjust the angle of their guide pin to best match these discrepancies in the planning phase.

For the recipient prep, it can be helpful to place the guide pin bicortically through the dorsal cortex of the patella for added stability. When using the scoring reamer and actual reamer, constant cold irrigation must be used to prevent frictional temperature increases, which has been demonstrated, with literature in press, to affect chondrocyte viability. ⁸ A similar constant cold irrigation technique should be used for the donor harvest and preparation. The exact angle must be taken to place the guide pin and ream at the same respective location on the donor patella to best match the defect. Given that the patella has a convex dorsal surface, when holding and reaming the donor graft, it is easy for the patella to tilt, causing a nonorthogonal reaming trajectory and resulting in poor donor-recipient matching. The combination of towel clips, blue towel padding, and assistants can combat this error.

Measurements are then taken on the recipient side, and we find 4 locations to be adequate. These should be recorded and done at a minimum of 2 times by 2 sets of eyes to minimize interrater error. These are then marked on the donor plug. In an ideal scenario, all measurements are the same, and a flat cut can be made perpendicular to the graft. However, in the patella, especially, there are often varying depths of measurements. In this case, a best-fit flat, orthogonal cut is still the best option, rather than undulating the bony cut on the donor to match the recipient depths exactly. This is because the recipient's differences in depth are occurring at the chondral surface; on the bone side, the reamer leaves a flat, deep surface. Attempting to match the varying depths of the donor with an undulating bony cut will leave the patient with bony gaps upon implantation, wherever the donor plug has a shorter depth. Even an oblique cut of the donor plug, if one side is longer than the other, will not compensate for an uneven recipient defect, as the longer side of the donor plug will make contact with the deep surface in the recipient before the shorter side of the donor plug, even with rotation of the donor plug.

Once the donor plug is prepared, several additional steps must be taken for optimal graft insertion and integration. The recipient site should be dilated, and care should be taken to insert the dilator perfectly perpendicular, as it is a highly dense metal cylinder that can shear adjacent cartilage with a poor insertion trajectory. The donor graft should also be beveled at the leading edge with a saw in a feathering motion, allowing for a reduced force required for a nonimpaction press-fit insertion. Graft integration should be optimized with an initial pulse lavage to irrigate out marrow elements, followed by a sterile carbon dioxide jet to further remove lipid elements and remaining fluid from the graft. This function decreases graft antigenicity, primarily from the subchondral bone, and increases donor plug porosity for potential orthobiologic uptake, as demonstrated in this study. A study in Cartilage demonstrated significantly reduced bone marrow elements after the combination of saline plus CO₂ lavage, compared with controls or pulse lavage alone. ⁷ When ready for graft insertion, it is essential to insert the graft perpendicularly to the defect to allow for a reduced force, nonimpaction insertion technique. The graft can be held in place with a rescue suture tape in a manner that allows for perpendicular insertion. If necessary, a freer can be used as a shoehorn to guide placement of the OCA. Using only thumb manual pressure is sufficient if the recipient and donor have been prepared adequately. Avoiding mallet impaction is essential, as both increased peak impaction force and repetitive lower impacts have negative effects on chondrocyte viability. ⁵

The final construct should have no step-offs or incongruency with the surrounding recipient bed. As a final step, prominent edges on either the recipient or donor side can be beveled with a single cut of a 15-blade.

Results

There are limited long-term outcome series in the literature on isolated patellofemoral OCAs, with 1 study demonstrating excellent survivorship of OCA plugs in the patellofemoral joint up to 8 to 10 years, with drastically lower survival rates with large shell-type OCAs, and the senior author has moved to utilizing larger plugs and leaving some residual defect, rather than using a true, shell allograft. ²

There’s a paucity of comparative literature on the OCA versus surface-based treatment for patellofemoral chondral lesions. Two recent studies, published in 2025, compared surface-based lesion treatment and OCA in small cohorts for the patellofemoral joint. Both studies demonstrated significant improvements in patient-reported outcomes (PROs) in both groups with comparable complication rates and reoperation rates after both treatments in the patellofemoral joint, with a slight advantage in improvements in PROs toward ACI and MACI procedures.^3,9 Systematic reviews and meta-analyses of patellofemoral cartilage restoration have demonstrated significant improvements in all PROs after all treatment options in patellofemoral restoration. Two recent systematic reviews by Chahla et al ¹ and Hinckel et al ⁴ demonstrate a weighted 5-year mean survival rate of 87.9%, a 10-year mean survival rate of 77.2%, and a 15-year mean survival rate of 55.8% after osteochondral allografting of the patellofemoral joint.

Discussion/Conclusion

Patellar OCA transplantation is a complex procedure that requires preoperative planning, intraoperative techniques, and postoperative considerations. Ensuring adequate matching between donor-recipient graphs, using technical pearls that optimize donor-recipient congruity, and implementing steps to maximize integration in graph biology can help achieve optimal outcomes.