Dx: Right hip OA with referred knee pain
The patient’s history and physical exam prompted us to suspect right hip osteoarthritis (OA) with referred pain to the right knee. This suspicion was confirmed with hip radiographs (FIGURE 2), which revealed significant OA of the right hip, as evidenced by marked joint space narrowing, subchondral sclerosis, and osteophytes. There was also superior migration of the right femoral head relative to the acetabulum. Additionally, there was loss of sphericity of the right femoral head, suggesting avascular necrosis with collapse.
Hip and knee OA are among the most common causes of disability worldwide. Knee and hip pain are estimated to affect up to 27% and 15% of the general population, respectively.1,2 Referred knee pain secondary to hip pathology, also known as atypical knee pain, has been cited at highly variable rates, ranging from 2% to 27%.3
Eighty-six percent of patients with atypical knee pain experience a delay in diagnosis of more than 1 year.4 Half of these patients require the use of a wheelchair or walker for community navigation.4 These findings highlight the impact that a delay in diagnosis can have on the day-to-day quality of life for these patients. Also, delayed or missed diagnoses may have contributed to the doubling in the rate of knee replacement surgery from 2000 to 2010 and the reports that up to one-third of knee replacement surgeries did not meet appropriate criteria to be performed.5,6
Convergence confusion
Referred pain is likely explained by the convergence of nociceptive and non-nociceptive nerve fibers.7 Both of these fiber types conduct action potentials that terminate at second order neurons. Occasionally, nociceptive nerve fibers from different parts of the body (ie, knee and hip) terminate at the same second order fiber. At this point of convergence, higher brain centers lose their ability to discriminate the anatomic location of origin. This results in the perception of pain in a different location, where there is no intrinsic pathology.
Patients with hip OA report that the most common locations of pain are the groin, anterior thigh, buttock, anterior knee, and greater trochanter.3 One small study revealed that 85% of patients with referred pain who underwent total hip arthroplasty (THA) reported complete resolution of pain symptoms within 4 days of the procedure.3
Continue to: A comprehensive exam can reveal a different origin of pain