DIFFERENTIAL DIAGNOSIS

The differential diagnosis of the painful THA can be organized into two divisions: causes that are intrinsic to the hip and causes that are extrinsic to the hip (Table 30-1). This comprehensive list of conditions can be used as a starting point for the workup.

TABLE 30-1 DIFFERENTIAL DIAGNOSIS OF THE PAINFUL TOTAL HIP ARTHROPLASTY

HISTORY

A detailed history and careful examination are important first steps in the evaluation of the painful THA. This information can be used to narrow the differential diagnosis considerably, leading to a more efficient and cost-effective workup. The history should focus on the various aspects of the patients’ pain complaint: its onset, location, timing, and palliative and provocative factors.

Pain that continues to be exactly the same after surgery as before the surgery suggests a possible extrinsic cause for the hip pain that was not addressed by the THA. On the other hand, pain that is different from the preoperative pain and that starts immediately after the surgery suggests an early postoperative complication, such as unrecognized intraoperative fracture, dislocation, poor fixation leading to early loosening of the implant, acute infection, or hematoma. Pain that starts to occur much later, after a long pain-free interval, is more consistent with aseptic loosening, osteolysis, periprosthetic fracture, late infection, bursitis, or tendonitis.⁵ Late pain can also be due to an extrinsic cause, exacerbated by the increased activity that a successful THA typically allows.

The location of the pain can be very helpful, although some patients find it hard to precisely localize their symptoms. Pain felt in the groin or deep buttock is consistent with problems with the acetabular component. However, groin pain is also associated with inguinal, femoral, or obturator hernia⁶; iliopsoas tendonitis due to acetabular retroversion and impingement⁷; and a number of genitourinary and gynecologic diseases. Moreover, pain in the buttock could be from referred low back pain, sacroiliac disease, or piriformis syndrome.

Pain over the greater trochanter is frequently due to bursitis, sometimes related to underlying sutures or wires, but can also be from trochanteric fracture or nonunion.

Anterior thigh pain is associated with femoral stem loosening, as well as mismatch of stiffness between the host bone and femoral stem.⁸ Thigh pain can also be due to neurogenic referred pain, especially posterolateral thigh pain that is numb and tingling in character and radiates below the knee in a dermatomal distribution. This is most likely from lumbar radiculopathy, but could also be from iatrogenic injury to the sciatic nerve.⁹ Finally, if the pain is in the calf, consider neurogenic or vascular claudication.

In regard to timing, pain at rest or at night is associated with infection or malignancy. In particular, constant mild pain after THA is suggestive of infection.¹⁰ Pain that is aggravated by activity and is relieved with rest could be from tendonitis or a loose component. In particular, pain that follows a triphasic pattern of initial pain on starting activity, followed by a lessening of pain, then increasing with prolonged activity, is consistent with a loose component in its early stages. Pain that occurs only in certain positions suggests subluxation or dislocation.

The patient should also be asked about any possible precipitating events. Pain beginning with a fall may be from a fracture or traumatic loosening. A recent distant infection, a dental, gastrointestinal, or genitourinary procedure, a systemic illness, and wound healing problems, including postoperative hematoma or persistent drainage, raise the possibility of infected THA. Past medical history can also reveal factors that increase the chance of infection, such as diabetes, obesity, chronic inflammatory conditions, immunocompromised status, and previous hip surgeries.¹¹

PHYSICAL EXAMINATION

The physical examination should include a thorough assessment of both hips, both knees, and the spine, to assess both the painful hip as well as other potential sources of referred pain. Starting in the standing position, one should inspect the spinal alignment for scoliosis, kyphosis, lordosis, or pelvic obliquity, and look for gluteal, hamstring, or quadriceps muscle atrophy. Single-leg stance should be tested for several seconds to see if Trendelenburg sign will turn positive; this sign is consistent with abductor weakness, trochanteric nonunion, gluteal nerve injury, adduction deformity, ankylosis, as well as THA failure. Standing leg length measurements should be performed with blocks to determine true versus apparent leg length discrepancy. A gait examination is then done, both with and without walking aids, if applicable, to check for antalgic gait or Trendelenburg gait and to assess balance and mobility.

Moving then to a supine position, inspection should be done to check for distal muscle atrophy or asymmetry, overall limb alignment, and supine leg lengths. Skin examination is done to look for dermatitis, cellulitis, abscesses, ulceration, thin skin, or chronic or active sinuses. Palpation about the hip may reveal tenderness over the trochanter in trochanteric bursitis or nonunion, over the sacroiliac joint for sacroiliitis, over the ischium for hamstring tendonitis, over the pubic rami for pelvic stress fracture, over the gluteus maximum insertion site, or over the spinous processes. Palpation can also reveal fascial defects and masses due to hernia, heterotopic ossification, or ectopic cement.

Range of motion of the hip is tested through both active and passive arcs, and any painful response is noted. Pain only at the extremes of motion can be seen with loosening, whereas pain throughout the arc of motion can be seen with active synovitis or infection.¹⁰ Stability examination must be performed very carefully lest frank dislocation occur in the office setting. Strength examination may reveal weakness in pure flexion, which could occur in the setting of iliopsoas tendonitis or previous psoas tendon release, or weakness in abduction, from abductor tendonitis or avulsion of the gluteus medius tendon.

Because one of the more common extrinsic differential causes of hip pain is referred pain from a primary pathologic process of the back, the back and neurologic examination is an important component of the workup for painful THA. Careful attention should be given to the sensory, motor, and reflex function of the femoral nerve, the obturator nerve, and both common peroneal and tibial branches of the sciatic nerve. Stretch tests can be performed on the sciatic and femoral nerves. Attention should be given to possible dermatomal or myotomal patterns of dysfunction. Finally, the vascular and lymphatic systems should also be evaluated by noting distal pulses, atrophic skin changes, or peripheral edema.

LABORATORY TESTS

Peripheral white blood cell count (WBC), erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) levels are commonly used in the workup of the possibly infected THA. At this time, it is generally acknowledged that the WBC has little utility in this setting, owing to its low sensitivity. Canner and coworkers reported that in only 15% of 52 known cases with infected THA was there an elevated WBC.¹¹ In addition, Spangehl and associates found in a prospective study of 178 patients having revision THA that the WBC had only sensitivity of 20% for infection.¹²

The ESR, on the other hand, is a very sensitive marker for inflammation. It increases after THA and returns to less than 20 mm/hr within 6 months.¹³ Forster and Crawford reported that their group of patients with established infected THA started with a mean ESR of 60 mm/hr, which then dropped to below 20 mm/hr after the infection was eradicated. Sensitivity of the ESR to infection has been reported at between 78% and 100%.¹⁴ However, the ESR is not very specific and can be elevated in rheumatoid arthritis, connective tissue disorders, malignancy, and pregnancy.

More helpful even is the CRP, because it returns to normal within 3 weeks postoperatively.¹⁵ Sanzen and Carlsson found that only 1 of 23 infected hips, but all 33 infected hips, had a CRP less than 20 mg/L.¹⁶ Like the ESR, it is a very sensitive marker for inflammation, but with a very low specificity.

The best use of these highly sensitive tests may be to rule out infection if they are both normal. Spangehl and associates used the combination of both ESR and CRP to achieve 100% specificity for excluding the diagnosis of infected THA.¹²

ASPIRATION

Aspiration is accepted as an important tool in the workup for painful THA. However, its routine use before revision THA is more controversial. Roberts and associates reported sensitivity of 87% and specificity of 97% in their series of 78 patients who underwent revision THA.¹⁷ Levitsky and coworkers reported an even higher sensitivity of 92% and specificity of 97% when combined with an elevated ESR and a postoperative interval of less than 5 years.¹⁸ However, Barrack and Harris have been widely cited as finding, in a large series of 270 cases, a high false-positive rate of 13%, with only 6% positive predictive value, and have recommended against the use of aspiration routinely before every revision THA.¹⁹ Several authors have advised its use only after preliminary laboratory test results or nuclear imaging test results are abnormal, to confirm the diagnosis and identify the pathologic organism in question.^5,10,14