Hip OA typically presents as pain associated with limitations in walking and climbing stairs. The pain is usually deep within the hip, but can be in the groin or thigh and can radiate to buttocks or to the knee. Hip OA can even present as low back pain. Pain and stiffness is generally worse first thing in the morning and after sitting or resting. The pain may ease with gentle exercise but worsen with vigorous exercise. Occasionally the joint may seem to lock or ‘stick’, and crepitus may be felt during movement. Over time the range of movement may decrease, with difficulty achieving a full stride or reaching a high step. Pain walking leads to the classic ‘antalgic gait’ where patients try to minimise pain on weight-bearing by altering the forces through the hip and taking a shorter more rapid step on the affected side. Use of a stick of crutch on the alternative side distributes the load and reduces the muscular force applied through the joint. Standing on the affected side may lead to a drop of the contralateral hip either because of wasting of the hip abductors or to reduce force through the joint (Trendelenburg sign). Loss of hip extension may result in a corresponding increase in lumbar lordosis with associated low back pain. Loss of flexion and rotation makes it harder to squat and put on shoes and socks.
The underlying disease process in OA hip, common to all OA joint changes, involves degenerative changes in the articular cartilage. Loss of articular cartilage leads to exposure of the underlying bone. Altered mechanical forces on the bone leads to subchondral sclerosis, subchondral cyst formation and osteophyte formation at the joint margins. The loss of articular cartilage results in narrowed joint space on X-ray, and this combined with osteophyte formation provides the characteristic appearance of OA.
Diagnosis can be made on the basis of clinical findings but is confirmed by imaging studies. There is a lack of standardisation in diagnosis, particularly in assessment of progression which may be based only on worsening of self-reported pain or function. Typically diagnosis and assessment of progression may be by decreased radiographic joint space, an increase in they summary radiographic grade (typically Kellgren-Lawrence), increase in total osteophyte score. Ultimately progression is determined in many studies by total hip replacement (Foley et al., 2015).
To illustrate the difficulties in diagnosis, a study found hip pain in 7% of males and 10% females, while severe OA was present in 16% of those with and 3% of those without pain. While there was still a strong association of pain with severe OA (OR 17.4 95% CI 3.0-102), there was no association of pain with mild/moderate OA. Only 22% of men age 45-54 with severe OA had current pain, however the proportions with pain were higher in older age groups (54-70%) (Birrell et al., 2005).
Kellgren and Lawrence developed a radiographic classification in the 1950s which is generally used today. Based on four features, joint space narrowing, osteophyte formation, subchondral sclerosis and subchondral cysts, the following grades are described:
Grade 0. No radiographic features of OA.
Grade 1. Doubtful joint space narrowing and possible osteophyte lipping.
Grade 2. Definite osteophytes and possible joint space narrowing.
Grade 3. Multiple osteophytes, definite joint space narrowing, sclerosis and possible bony deformity.
Grade 4. Large osteophytes, marked joint space narrowing, severe sclerosis and definite bony deformity.
BIRRELL, F., LUNT, M., MACFARLANE, G. & SILMAN, A. 2005. Association between pain in the hip region and radiographic changes of osteoarthritis: results from a population-based study. Rheumatology (Oxford), 44, 337-41.
FOLEY, B., CLEVELAND, R. J., RENNER, J. B., JORDAN, J. M. & NELSON, A. E. 2015. Racial differences in associations between baseline patterns of radiographic osteoarthritis and multiple definitions of progression of hip osteoarthritis: the Johnston County Osteoarthritis Project. Arthritis Res Ther, 17, 366.