Review article explores latest research on three pathologic entities
Knee osteonecrosis (ON) is a progressive disease that can lead to subchondral collapse and debilitating arthritis. Researchers initially thought it developed spontaneously and typically involved the medial femoral condyle.
That view of the disease has expanded, however, to include three distinct pathologic entities: secondary ON, spontaneous ON, and postarthroscopic ON.
To find out how these disease entities differ and how they can be diagnosed and treated, AAOS Now spoke with Michael A. Mont, MD, the lead author of “Osteonecrosis of the Knee and Related Conditions,” a review article in the August issue of the Journal of the AAOS.
AAOS Now: What do we know about secondary ON?
Dr. Mont: Secondary ON often involves both femoral condyles, with multiple lesions in the epiphysis, metaphysis, and diaphysis of the bone. Patients are typically younger than 45 years. Secondary ON is bilateral more than 80 percent of the time.
Direct risk factors for secondary ON include radiation, chemotherapy, and trauma. Conditions such as sickle cell disease or other myeloproliferative disorders also increase the risk for secondary ON.
One indirect risk factor is a certain level of corticosteroid use, (ie, taking more than 1 gram per month of prednisone or equivalent for 2 or more months, or approximately 30 mg per day). In fact, secondary ON got its name because it was said to be secondary to corticosteroid use. Other factors that can lead to this disease include alcoholism and an inherited coagulation disorder.
We should remember, however, that ON develops in only about 10 percent of patients on high-dose steroids and in a much lower percentage of alcoholics.
AAOS Now: What about the other two types of the disease—spontaneous ON and postarthroscopic ON?
Dr. Mont: We know much less about spontaneous ON, in which epiphyseal lesions occur in either the medial or lateral femoral condyle, or in the medial or lateral tibial plateau. Patients are typically older than 50 years and three times more likely to be female. Almost all cases are unilateral. Spontaneous ON may result from a subcortical insufficiency fracture that causes the bone to become osteopenic, which leads to the lesion. Some histologic studies have found osteopenia or osteoporosis in patients with spontaneous ON.
Postarthroscopic ON is believed to result from the laser heating up the bone and killing it. Another theory suggests that trauma during surgery causes it.
Postarthroscopic ON is similar to spontaneous ON in that lesions are typically only found in the epiphysis. Patient age varies broadly, and sex is not a factor. Some of the associated risk factors include meniscectomy, cartilage débridement, and anterior cruciate ligament reconstruction.
AAOS Now: How is knee ON diagnosed? What is the role of magnetic resonance imaging (MRI)?
Dr. Mont: In the early stages, the lesions are not visible on radiographs about 80 percent of the time. Once the condition is visible on radiographs, it’s already at a later stage and the knee may already have collapsed.
The best diagnostic tool for all three disease entities is MRI, which enables the orthopaedist to detect lesions early by assessing marrow viability and lesion distribution and to evaluate meniscal and chondral pathology (Fig. 1).
Patients with knee ON who complain of minor pain in the hip should undergo a hip MRI scan to see if hip ON is developing. Similarly, patients with hip ON who start complaining of knee pain should have MRIs and/or radiographs of the knee. I don’t advocate using bone scans to diagnose knee ON.
AAOS Now: What are the treatment options for secondary ON?
Dr. Mont: If secondary ON is diagnosed in the early stages, an orthopaedist can usually drill the lesions, which may halt the progression of the disease. Some orthopaedists supplement the drilling technique with ancillary bone morphogenetic protein and growth factors to try to aid the healing process.
Bone grafting or osteotomy have been proposed as treatments, but because these lesions typically involve both condyles, they’re often not amenable to these methods. In patients with somewhat advanced knee ON, the orthopaedist may be able to open the knee and graft from the inside, assuming only one isolated area has collapsed. This may help avoid joint arthroplasty.
Nonsurgical treatment typically does not result in good outcomes in many patients with secondary ON; in many cases, the knee collapses and the patient requires arthroplasty. Fortunately, knee arthroplasty results today are much better, even in this young patient population, than they were in the 1980s and 1990s.
AAOS Now: What are the treatment options for the other two types of knee ON?
Dr. Mont: With spontaneous knee ON, treatment is often dictated by the size of the lesion. Small, isolated lesions often regress and heal. Medium-sized lesions may regress, or, as with very large lesions, subchondral collapse will occur, regardless of treatment. Typically, initial treatment involves nonsurgical measures, such as limited weightbearing, for small and medium lesions. Surgical options are used with large lesions.
If one condyle collapses and previous treatment has been unsuccessful, the patient might be an appropriate candidate for unicompartmental knee arthroplasty (UKA). If the disease progresses to arthritis affecting the entire joint, then total knee arthroplasty (TKA) may be appropriate. The results for UKA and TKA in this patient population have generally been excellent.
Most of the initial reports indicated that postarthroscopic ON resulted in partial or total knee arthroplasties because the knee would collapse. If the condition is caught early, grafting or other joint sparing techniques may be possible, but we don’t have much data on that. Typically, patients with postarthroscopic ON do fine with partial or total knee arthroplasties.
AAOS Now: What kind of research still needs to be done on knee ON?
Dr. Mont: We need more prospective, randomized studies, which may be difficult to conduct due to the small number of patients at any one center. The incidence of knee ON is about 10 percent of the incidence of hip ON; there are about 1,000 to 1,500 cases of knee ON annually, compared to about 10,000 cases of hip ON annually.
We should also keep exploring treatment methods that enable us to avoid performing more UKAs and TKAs and more studies on early diagnosis options, patient susceptibility, and biologic treatment methods.
Jennie McKee is a staff writer for AAOS Now. She can be reached at email@example.com