An osteochondral injury can occur in up to 71% to 93% of patients during an acute patellar dislocation.¹,²

The chondral and osteochondral injury can vary from minor cartilage scuffing or fissuring, to large full-thickness free fragments greater than 3 cm diameter or width.

Osteochondral fractures are usually generated during a patient’s first episode of patellar dislocation and occur in 15% to 29% of acute patellar dislocations in the pediatric population²,³,⁴,⁵

Though patellar dislocation usually occurs in early flexion (0°-30°), osteochondral fractures may occur when the knee is flexed about 90° to 135° because more force is required to dislocate patella in this position.⁶

As the patella is pulled laterally, the medial facet of patella would contact the lateral femoral condyle. This would lead to simultaneous bone bruises on the anterior half of the lateral femoral condyle and on the medial facet of the patella. Such contusions could be present in up to 100% of first-time dislocators in adults.⁴,⁷ This pattern of bone bruise is pathognomonic of patellar dislocation.

Most pure chondral fragments in children and adolescents usually contain a microscopic sliver of subchondral bone on their deep surface, so even apparent pure chondral lesions are amenable to repair. Mature cartilage tends to shear off at the junction between the uncalcified cartilage and the calcified cartilage layer (tidemark). Immature cartilage lacks a calcified cartilage layer and hence tends to sheer off deeper in the subchondral bone layer, as shown in a bovine model.⁸ A repairable osteochondral fracture serves as the primary indication for surgery on firsttime patella dislocators.⁹

Many osteochondral fractures in adolescents and young adults emanate from the patella, but osteochondral lesions from the lateral femoral condyle are reported in 28% to 40% of patellar dislocations.¹,²,¹⁰,¹¹,¹² The size, condition, and location of the fragment determine the necessity and feasibility of repair.

Medial patella avulsion fractures from the medial patellofemoral ligament (MPFL) (marginal fractures) occur 62% of the time in pediatric patients based on ultrasonography¹³ and 43% based on magnetic resonance imaging (MRI).⁶ These are usually nonarticular lesions. Though they help confirm the diagnosis of patellar dislocation, they do not require treatment¹⁴ (Figure 24.1).

These marginal avulsion fractures need to be differentiated from articular fractures of the patella and lateral femoral condyle, which often require surgery for removal or repair depending on their size and condition.

The International Cartilage Research Society (ICRS) is the favored classification system for cartilage surface injury, with grade I lesions being fissures, cracks, and indents that are superficial. Grade II lesions penetrate down to 50% of the cartilage depth, grade III penetrate greater than 50% of depth to the calcified cartilage, and grade IV lesions have exposed bone (Figure 24.2). Almost all patients with traumatic patellar dislocation will have at least ICRS grade I-II fissures over the medial patella and/or the lateral femoral condyle.¹⁵

Osteochondral injuries specifically associated with patellar dislocation have been classified.¹,¹⁶

Nomura et al described patterns of patellar cartilage lesions specific to recurrent patellar dislocation because the previously popular Outerbridge classification focused on chondromalacia patellae.¹⁵,¹⁶,¹⁷

According to Nomura et al, the types of patellar chondral lesions include fissuring, fibrillation, fibrillation with erosion, erosion, and convex cartilage formation (Figure 24.3). Fissuring was present in 76% of patients, and the most common site of fissuring was on the central dome. Fissuring was further categorized as multiple longitudinal fissures and marginal/radial fissures (Figure 24.4). Fibrillation or erosion or both were observed in 77% of patients, mainly over the medial facet.¹⁶

No formal classification exists specifically for osteochondral loose bodies that occur after patellar dislocation. Osteochondral fractures and loose bodies are more common than pure cartilaginous loose bodies, especially in younger patients. These “pure chondral” lesions may have no visible bone attached, but may have a gritty feel when probed. This may be indicative of a nonvisible microscopic layer of bone attached to the deep, nonarticular surface of the loose fragment, especially in immature patients.⁸,¹⁸

Hypermobile patients are less likely to have osteochondral fractures or the more benign medial patella marginal avulsion fractures as compared with nonhypermobile patients.⁴,¹⁹,²⁰

Table 24.1 shows the indications and contraindications for removal of an osteochondral fragment.

Most patients would confidently describe their patella moving in a lateral direction out of place, but a few would be clueless as to what happened. The noncontact mechanism with a rotational/flexion component is very similar to that of an anterior cruciate ligament rupture.

A history of significant effusion associated with patellar dislocation may suggest an associated osteochondral fracture.

A history of recurrent, atraumatic injury, and minimal or no effusion may suggest a patient with ligamentous laxity. This latter group is at much lower risk for osteochondral injury with a patellar dislocation, presumably because of the decreased shear forces on the patellofemoral joint surface as a result of lax ligaments.⁴,²⁰

Patients with a history of locking or catching after a patellar dislocation suggest an osteochondral loose body.

Osteochondral injury and fractures are most commonly seen with first-time acute patellar dislocation sustained after a traumatic event that produces a large knee effusion.⁴

Unfortunately, increased knee pain, effusion, and apprehension can limit the acute injury physical examination.

Effusion, patellar apprehension, and increased patellar lateral translation at 30° knee flexion compared with opposite knee suggest patellar instability.

The physical findings for an osteochondral injury or loose body in the setting of patellar dislocation have not been tested for reliability and are unlikely to be highly sensitive or specific. Because patellar stabilization is often performed simultaneous with osteochondral fragment repair or removal, the patient should undergo assessment of leg alignment, rotation, ligamentous laxity, and skeletal maturity during the preoperative physical examination.

Three-view knee x-ray images (anteroposterior [AP], lateral, and sunrise) supplement the patient history and examination. Most patients receive the AP and lateral x-rays in the emergency room or urgent care prior to their orthopedic visit. It is often impossible on the day of injury to obtain a sunrise view because of the high knee flexion angle required for this view.

If the sunrise view is missing, it should be obtained because this a key x-ray image. In addition to confirming patellar reduction, it can show the medial patella avulsion fracture that is near-diagnostic of a patellar dislocation and can show a lateral femoral condyle osteochondral fracture fragment in the lateral gutter. All the images should be scrutinized for an intra-articular osteochondral loose body (Figure 24.5).

These MPFL avulsion fractures are essentially nonarticular and do not need to be removed or repaired¹⁴ (Figure 24.1).

Osteochondral fracture can be missed on plain x-ray in 44% to 71% of adolescents’ patients.¹,²,¹⁰ MRI and/or computed tomography (CT) scanning should be considered in most young patients after a patellar dislocation associated with a large joint effusion/hemarthrosis to identify size and to locate and estimate the condition of these fractures.

High-quality MRI performed on a 1.5- or 3.0-T magnet can generate images of cartilage images that mirror those seen at arthroscopy. MRI is highly reliable in the identification of osteochondral injuries.²¹

Most traumatic patellar dislocations generate MRI evidence of articular cartilage damage, including superficial longitudinal fissures, fibrillation, flap tears, and voids. Many of these are minor and may not need surgical treatment.

MRI is essential for the recognition of significant osteochondral fractures that may need surgical repair or removal to improve long-term outcomes. The medial patella and the lateral femoral condyle need to be evaluated for articular surface voids. In the acute situation, the knee effusion highlights these articular surface defects on MRI (Figure 24.6).

Once an articular void is identified, the MRI is carefully evaluated to search for a loose body. Similarly, if a loose body is identified on the MRI, the source of the loose body is carefully evaluated.

Loose bodies can be hidden in the posterior compartments, under the anterior horn of the medial or lateral meniscus or in the suprapatellar pouch.

The MRI has supplanted several other studies that can be useful on occasion. The knee can be aspirated for fat globules, which can indicate an osteochondral fracture, and high inflammatory markers in the aspirate can also suggest osteochondral injury.²² Ultrasound
can reliably identify loose bodies and osteochondral injury after patellar dislocation.¹³,²³