Background
Focal articular cartilage lesions or defects are common in arthroscopic surgery of the knee. Chondral defects may occur as a result of overt trauma, but can also occur because of a de novo process such as osteochondritis dissecans (OCD). Although it is thought that OCD affects the subchondral bone, these lesions can become unstable and thus involve overlying cartilaginous changes. Epidemiologic data on articular cartilage lesions in the pediatric or juvenile population are not clear and vary depending on such factors as body mass index, type of sport, family history, traumatic versus nontraumatic injury, and so on.
In 2004, Oeppen et al. reported chondral injuries occurred in 34% of skeletally immature subjects who had sustained a traumatic event to the knee with suspected internal derangement. The majority of these chondral injuries were located on the femur. In 2013, Kessler et al. used a large integrated health system database of over a million patients between the ages of 6 and 19 years and determined the incidence of OCD of the knee to be 3.3 and 15.4/100,000 for females and males, respectively. In 2017, Ellermann et al. linked juvenile OCD lesions to epiphyseal cartilage necrosis, suggesting there may be a natural progression from epiphyseal cartilage origin to secondary progeny ossification in some OCD occurrences.
Numerous treatment algorithms for addressing OCD injuries exist, including techniques such as drilling of the lesion, osteochondral grafting, fixation with hardware, autologous chondrocyte implantation, and other cell-based therapies. Achieving good outcomes depends on several factors, and even in the most experienced hands complications can still arise. The focus of this chapter will be to discuss the preoperative considerations for the aforementioned techniques, the intraoperative issues that may arise, and how these can be managed. Case examples will be used to illustrate the most germane and technically challenging techniques.
Preoperative Considerations
Clinical Exam
Patients who have sustained a focal cartilage injury can present with a triad of activity-related pain, intermittent knee swelling, and catching or locking. A thorough physical examination is important because this will help narrow the diagnosis. A commonly referenced clinical examination is Wilson’s test, whereby the tibia is internally rotated and brought from 90 degrees of flexion to full extension to detect OCD of the medial femoral condyle of the knee. Clinically this test may have poor reliability, as cartilage injury can occur elsewhere within the knee such as the lateral condyle, patella, or trochlear sulcus, which are not evaluated in Wilson’s test. As such, this clinical examination has fallen out of favor.
Imaging
Although magnetic resonance imaging (MRI) remains the gold standard imaging modality for evaluating cartilage, most authors recommend plain radiographs for initial radiographic evaluation of the injured knee because of the relative ease and cost, as well as its ability to provide critical information about factors such as limb alignment. Initial radiographs should include the anteroposterior, lateral, sunrise, and Rosenberg views in addition to a full-length lower extremity radiograph such as a scanogram. MRI is important for characterizing abnormalities in the cartilage and subchondral bone, and has the added value of evaluating soft tissue injury. , Chronicity of the injury can be inferred by signs seen in the subchondral bone such as sclerosis, cysts, and the extent and intensity of the bone marrow edema adjacent to the lesion. Specific MRI protocols have been published to better visualize OCD lesions. Bohndorf et al. recommend a T1 weighted, turbo spin echo (TSE) sequence with an additional T2-weighted TSE, gradient echo sequence to assess overlying cartilage, and a short-time inversion recovery sequence for viewing edema. More recently, several sequences have been developed and are showing promise in evaluating cartilage. These sequences include delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), T1 Rho, sodium MRI, and T2 mapping studies. , Ellermann et al. further specify this imaging protocol by identifying the benefits of using the shortest echo time attainable (ideally 4 msec) on a clinical imaging unit. This imaging sequence allows for visualization of a type 1 “entirely cartilaginous lesion,” that is, the earliest stage of OCD development. However, because of concerns over gadolinium saturation in the cartilage, dGEMRIC has more functional utility in a research setting than it does clinically.
Alignment Considerations
Limb and joint malalignment play a role in the development and healing potential of cartilage defects. Obtaining a full-length radiograph such as a scanogram or an EOS image can be beneficial in preoperative planning ( Fig. 21.1 ), as varus or valgus malalignment may contribute to a poor mechanical environment for healing depending on the location of the lesion. In some cases, the use of “guided growth” or corrective osteotomy may be part of the treatment plan to obtain a more normal mechanical or anatomic axis of the extremity.
Autologous Chondrocyte Implantation/Matrix-Induced Autologous Chondrocyte Implantation
One of the options to consider for repair of chondral defects is autologous chondrocyte implantation (ACI), or its third-generation form, matrix-induced autologous chondrocyte implantation (MACI) (Varicel Corp, Cambridge, MA). Briefly, MACI involves growing the patient’s cultivated chondrocytes on an absorbent collagen membrane. This membrane is then placed within a prepared defect. Although the US Food and Drug Administration (FDA) states that safety and effectiveness have not been established in pediatric patients or patients over the age of 55 years, this procedure has been performed in patients under the age of 18 years in the United States and other countries. Preoperative considerations for ACI include bovine allergy. A documented alpha-gal allergy is a contraindication for ACI, as the cells are cultured in bovine serum, which the patient’s cells may not grow in if such an allergy is present. Anecdotally, at least one of the authors (AT) has encountered a patient who rejected the ACI graft twice and was subsequently found to have an alpha-gal allergy upon immunological testing. A thorough history and documented allergy review are important preoperative considerations when determining a young patient’s candidacy for this procedure.
Intraoperative Issues
Drilling
In situ drilling of stable OCD lesions can be done in an anterograde (transarticular) or retrograde (retroarticular) , fashion in a proximal to distal fashion to preserve the articular cartilage. Both drilling techniques should be done with fluoroscopic guidance in multiple planes to ensure appropriate placement of drill pathways. This is particularly important when drilling in an anterograde or retrograde fashion, as depth of drilling is often stopped prematurely to avoid violating the articular cartilage. Steps to reduce the amount of articular cartilage violated in a retrograde transarticular approach include using a smaller diameter K-wire such as 1.1 mm or 1.6 m ( Fig. 21.2 ) and angling the K-wire in multiple varying direction through the same transarticular hole. Care should be taken while redirecting the trajectory of each drill pathway to avoid bending the K-wire because this can result in intraarticular breakage of the K-wire during drilling. The surgeons may also consider drilling through the notch to avoid the articular cartilage, which may work for some lesions.
Fixation
Intraoperative fixation of the OCD can be achieved through a variety of implants. Herbert and Fisher first described the use of a “double-threaded bone screw” in the fixation of scaphoid fractures in 1984. These variable-pitch screws were then described for the use of osteochondral injuries of the femoral condyles by Wombwell and Nunley in 1987, with the specific advantage of allowing for compressive headless screw fixation, thus decreasing the likelihood of a proud screw head, which can lead to overlying cartilage injury. Care must be taken to ensure that there is adequate purchase in the subchondral bone, or the screw may back out, causing adjacent cartilage injury on the patella or tibial plateau. If the OCD fragment is fixed arthroscopically, it is prudent to secure the screw (with sutures over the shaft of the screw to keep this engaged on the screw drive) to the screwdriver before placement within the joint. Loss of the screw within the fat pad has been described, causing increased radiographic exposure for retrieval. Furthermore, care must be taken when driving the screw into the fragment and subchondral bone, because excessive, sudden torque may fracture the OCD fragment and even the screw. , If the OCD lesion becomes fragmented from overcompression of the screw, further attempts at fixation will likely yield an unsatisfactory result, requiring alternative treatment methods such as an osteochondral autograft or a fresh allograft transfer.
Metallic screws require a second procedure for removal. To eliminate the need for screw removal, fixation with bioabsorbable screws, darts, or nails has also been described. , An additional advantage of bioabsorbable implants is the avoidance of artifact on subsequent MRI. Despite the theoretical advantages of bioabsorbable implants, many cartilage surgeons have abandoned these because of complications such backout and cyst formation during dissolution of the implant, especially with threaded devices. Use of either implant requires careful follow-up, and both may require secondary surgery for removal or to address other issues. Absorbable implants however do leave a smaller footprint, which may be offset by the need for additional implants rather than a single screw.
Another factor to consider with screw use is the choice of an Allen or six-headed hexagon driver versus a star driver. One author has noticed a higher risk of screw stripping with Allen-head screws, and now uses star-driver screw heads for all cases. Titanium screws may be advantageous because they produce less MRI artifact compared with stainless screws. Headless screws with a diameter of 3.0mm, 3.5mm, or 4.0mm may be ideal for these lesions.
Careful intraoperative consideration of fixation for ex situ lesions should involve a thoughtful evaluation of remaining subchondral bone on the progeny fragment. Milgram et al. showed that only 50% of ex situ fragments had subchondral bone attached. Most agree that a minimum of 2 to 3 mm of subchondral bone must be intact to achieve adequate fixation of an osteochondral progeny fragment. However, some have suggested reasonable outcomes with fixation of fragments with inadequate subchondral bone. ,
Osteochondral Grafts
Osteochondral grafting can either consist of autograft transfer or a transfer from an allograft donor. D’Aubigne first described a technique of osteochondral autograft transfer in 1945, and the technique was later refined by Outerbridge et al. There are several limitations to using autograft transfer techniques. If the lesion is larger than 1 to 2 cm in diameter (2–4 cm 2 ) one may need to consider alternative techniques (e.g., osteochondral allograft, MACI with bone grafting). This limitation in size is owing to availability of donor site volume, especially in the pediatric population. Sherman et al. point out that, although the indications for treatment may be lesions larger than 1 cm, it is important to consider the context in individuals with smaller condyles, as a lesion smaller than 1 cm may be proportionally large (e.g., pediatric knee volume).
For both osteochondral allograft and autograft procedures, it is critical to identify concomitant intraarticular pathology and address this at the time of graft surgery. These associated injuries can include ligament instability, meniscal deficiency, or meniscal tears. Case 1 illustrated in Fig. 21.3 highlights a 16-year-old male with a large osteochondral lesion of the femoral condyle that failed rigid fixation and was revised to an osteochondral allograft; however, a concomitant meniscal tear was not addressed, resulting in failure of the allograft.
