Differentiating Hip Versus Spine Pathology
When a patient presents with pain following total hip arthroplasty (THA), it is essential to remember that pain patterns from lumbar spine disorders and hip pathology overlap. Overlapping pain patterns, coupled with the coexistence of lumbar spine disorders in 18% of patients undergoing THA, pose a challenge in the diagnosis of pain following THA. A thorough clinical history, physical exam, appropriate imaging, and diagnostic injections can help define the source of the patient’s pain.
Classically, radiculopathy is associated with pain that radiates past the knee, with intraarticular hip pain confined to the groin. Unfortunately, patients do not always present classically, with up to 47% of patients with hip arthritis indicated for THA reporting pain below the knee. Although groin pain has been reported to have a sensitivity of 84% and specificity of 70% for hip osteoarthritis, it is important to keep in mind that the following extraarticular sources can also generate groin pain: lumbar radiculopathy, facet syndrome, sacroiliac joint pain, and piriformis syndrome. A clear description of the pain distribution and exacerbating or alleviating factors will often steer diagnosis in the appropriate direction.
Spinal stenosis typically presents with neurogenic claudication described as back and bilateral lower extremity pain worsening with ambulation and improved with stooped posture or sitting. Physical exam findings in these patients are variable, with less than 20% of patients with spinal stenosis demonstrating a positive straight leg raise test or femoral tension sign. Rarely will the patient present with a neurologic deficit. These may include a positive Romberg test or weakness in coincident lower extremity distributions. Foraminal stenosis, specifically at L1 or L2, may result in groin pain. Foraminal stenosis due to a disc herniation or facet arthropathy typically worsens with increased lumbar lordosis in a standing position and improves with stooped posture or sitting. A thorough exam with vascular exam, reflexes, strength testing, sensation to pinprick, and pathologic reflexes, including clonus and Babinski sign, must be completed. Despite an exam, however, most physical tests to identify lumbar disc herniation and associated radiculopathy have low sensitivity and specificity.
Clinical history is invaluable when the possible etiology of pain is related to a patient’s prior hip arthroplasty. Pain in the groin or buttock can be related to acetabular-sided etiologies or infection while thigh pain is traditionally associated with femoral-sided loosening. Gait exam should evaluate for limb-length discrepancy and a Trendelenberg gait. In the acute perioperative period, the wound should be examined for any sign of infection. Later, the peri-incisional area should be evaluated for areas of erythema, warmth, or fluctuance. Active and passive range of motion should be assessed for pain and instability. Painful passive range of motion is concerning for infection, whereas apprehension is concerning for subtle instability.
Evaluation for Periprosthetic Joint Infection
Prosthetic joint infection (PJI) of the hip is a rare event, with 1- and 5-year incidences of 0.86% and 1.03%, respectively. Although uncommon, this diagnosis can be devastating to the patient and must always be considered when a patient presents with pain following THA. The first step in evaluating a patient for PJI is a clinical history. The level of concern for PJI should be increased if the patient had no pain-free period following THA or describes the acute onset of hip pain associated with a known inciting event such as a dental visit, break in the skin, or urinary tract infection. The presence of fevers and increased pain in the absence of trauma further raises concern. The physical exam in the perioperative period may be pertinent for wound erythema or drainage. Beyond the perioperative period, the exam may reveal a sinus tract, erythema, or fluctuance. The patient will often demonstrate an antalgic gait and pain with both active and passive range of motion.
Standard orthogonal radiographs of the involved hip should be completed and closely examined for any evidence of loosening or of an alternate pain etiology, such as fracture or dislocation. Laboratory evaluation should include serum C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR). These tests have excellent negative predictive value such that if both tests are without elevation, further infectious workup is typically unnecessary. These inflammatory markers are cost-effective and highly sensitive, with a threshold of 9.5 mg/L resulting in 85% sensitivity for PJI. In the first 3 weeks following surgery, these may be elevated due to surgical stress or inflammation but can still be a helpful data point during this time. Following THA, CRP is expected to peak on postoperative day 2 and return to normal by 3 weeks postoperatively. If CRP remains elevated after 3 weeks postoperatively or demonstrates a bimodal pattern of elevation, then the diagnosis of PJI must be considered. If clinical suspicion is high or the patient’s ESR or CRP is elevated, a hip aspiration should be performed. Synovial fluid should be sent for culture in addition to WBC count and percentage of polymorphonuclear (PMN) leukocytes. Cut-offs defining PJI after six weeks postoperatively are >3000 cells/microL and >80% PMN leukocytes. Early postoperative PJI within 6 weeks from surgery is consistent with the following markers: CRP >100 mg/L, synovial WBC count >10,000 cells/μL, and >90% PMN leukocytes. ,
Additional testing of synovial fluid can be performed if these results do not yield a definitive diagnosis. Synovial alpha-defensin can be a useful data point but should be considered in the context of the aforementioned labs, exam, and clinical history. The sensitivity of this assay was previously reported as greater than 95%, but more recent studies have called this into question, with lower sensitivity (78%) shown in the most recent prospective analysis. PCR gene sequencing of synovial fluid can also be used on synovial fluid with a sensitivity of 80% and specificity of 94%. If no fluid is able to be aspirated despite radiographic confirmation of intraarticular needle placement, then saline lavage of the hip can be completed. This involves injecting 20 mL normal saline and re-aspirating. This will dilute the WBC count, but the percentage of PMN leukocytes can still be helpful when this technique is utilized. With at least 80% PMN leukocytes on the differential, the sensitivity and specificity using this technique for diagnosis of PJI are 75% and 95%, respectively.
Ruling out infection plays a crucial role in the diagnosis of the etiology of pain in THA. Despite the presence of fracture, loosening, dislocation, or any other pathology, however obvious, the aforementioned workup for infection must be performed.
Osteolysis and Wear
Wear properties of polyethylene have dramatically improved with the advent of highly cross-linked polyethylene (HXLPE) as well as with improvements in sterilization and processing of polyethylene. Despite improvements, patients still present with HXLPE wear as well as traditional polyethylene wear from a remote THA. A patient with early polyethylene wear is most commonly asymptomatic but may present with particle-induced synovitis. As osteolysis progresses, resulting in component loosening, patients may endorse start-up pain or other hallmarks of loose components. As osteolysis becomes more advanced, patients may present with subluxation or dislocation due to asymmetric wear of the liner and soft-tissue laxity secondary to recurrent effusions. Additionally, these patients may fracture through lytic areas.
When imaging patients with possible osteolysis, there are a few considerations to keep in mind. The telltale sign of osteolysis on plain radiographic imaging is a femoral head that is sitting eccentrically within the acetabular component. The femoral head will specifically appear in closer proximity to the acetabular component or host bone in the presence of an all-polyethylene component, superiorly. Concern for pain from osteolysis should be considered when an eccentrically located head is found. Bony destruction from osteolysis is often found in conjunction with an eccentrically worn polyethylene. Plain radiographs often underestimate the amount of destruction from osteolysis; obtaining axial imaging is sometimes helpful.
If the patient presents with asymptomatic osteolysis, it may be appropriate to observe with serial radiographs every 6 months. Patients must understand that they will need to be followed radiographically and that without surgical intervention, osteolysis is not a reversible process. Surgery is considered once the patient develops symptoms or demonstrates radiographic loosening or worsening bone loss. Once the decision to re-operate on the hip is made, a plan must be considered to perform an isolated head and liner exchange or to also revise the acetabular implant ( Fig. 24.1 ).
To retain the acetabular implant, the following criteria should be met. The cup must be well fixed, which must be tested intraoperatively after the removal of all screws. The existing acetabular shell must be large enough to place a new liner with an appropriate femoral head size. Additionally, the cup must be positioned appropriately, with 35 to 50 degrees of inclination and 5 to 25 degrees of anteversion. Finally, the new liner must be stable within the shell and the hip stable through range of motion. A stable liner can be achieved by engaging the new liner in an intact locking mechanism or, if the locking mechanism is damaged, cementing a liner within the existing shell. If all of the preceding criteria are met, then a head-and-liner exchange with debridement and grafting of the lesions is appropriate. Lytic lesions may be accessed through a posterosuperior bone window or through the screw holes or through burred areas of the shell. The graft may consist of cancellous bone chips, calcium phosphate, or bone cement. Although the osteolytic lesions should be addressed with these aboe measures, the critical part of this procedure is addressing the liner, the particle wear generator. If all of the aforementioned criteria are met, then an isolated head-and-liner exchange, as compared with acetabular revision, will result in lower morbidity, lower bone loss, faster patient recovery, less blood loss, and decreased surgical time.
If prior stated criteria are not met, then the acetabular shell should be revised. This allows for total access to debride lytic lesions, repositioning the cup into an ideal position as well as the ability to increase the femoral head size. Compared with head-and-liner exchange, acetabular component revision is associated with increased blood loss, bone loss, and surgical time. In patients for whom a clear decision on shell retention cannot be made with the aforementioned criteria, it is important to consider patient factors, including age and comorbidities, to help weigh the risks of surgical morbidity associated with performing head-and-liner exchange versus revising an acetabular implant.
Recognizing Implants and Bearing Surfaces as Pain Generators
The implant record and prior operative report should be obtained and reviewed for any implants with known poor track records. Most notably, stems with a modular neck/body junction have reports of increased failure compared with both monobloc and implants with a modular stem/body. , Although modularity allows a surgeon to independently adjust version, length, and offset, each modular junction is susceptible to failure by corrosion and fretting. Not only can this lead to metallosis but more dramatically may result in implant dissociation at the neck/body junction, resulting in a well-fixed stem with a dissociated neck.
Other less commonly utilized bearing surfaces can be identified from the implant record. Ceramic-on-ceramic bearings have been used in attempts to avoid metal ion production, polyethylene wear, and adverse local soft-tissue reactions. Ceramic-on-ceramic bearings are particularly attractive for use in young patients, as they have the lowest wear rate; with ceramic debris being biologically inert, it does not incite the macrophage-triggered osteolytic process that polyethylene particles initiate. Despite these advantages, ceramic-on-ceramic bearings can be associated with painless squeaking and catastrophic ceramic fracture. Most commonly, these are liner fractures, but there have been reports of ceramic femoral head fracture as well. Contemporary ceramic bearings have a fracture rate of 0.2% to 0.5%, which may be attributed in part to asymmetric seating of the liner due to deformation of the cup or surgeon hesitancy to impact the ceramic with adequate force. When a patient is known to have a ceramic-on-ceramic bearing, fracture of the acetabular and femoral components must remain under consideration as a cause of pain ( Fig. 24.2 ).