How Should Surgeons Work Up a Patient with Instability Following THA?
Dr. Ajit Deshmukh answers ICJR’s questions about the causes of instability/dislocation in primary total hip arthroplasty patients, how surgeons can prevent instability, and when they should consider a revision procedure.
ICJR: What are the typical causes of instability/dislocation in primary total hip arthroplasty (THA)?
Ajit Deshmukh, MD: Causes of instability after primary THA can be categorized as:
Suboptimal component positioning. This is perhaps the most common and important reason for instability after primary THA. The relationship of acetabular and femoral component position to each other and to the patient’s native anatomy is very important.
Patient–related factors. These include:
- Neuromuscular disorders like Parkinson’s disease
- Cognitive problems, alcoholism
- Abductor deficiency
- Age over 80 years
- Female gender
- Spinal deformity and disease
- Non-compliance with precautions
- THA for hip fracture or hip dysplasia
- Prior open hip surgery, such as fracture fixation, osteotomy, hip fusion, or resection arthroplasty
- Significant trauma or fall with the extremity in an awkward position
Implant–related factors. Well-known implant-related causes of instability include:
- Small femoral heads
- Skirted femoral heads
- Suboptimal head-neck ratio
- Impingement due to elevated rim liners
- Metal ion-related adverse local soft tissue reaction from certain bearing surfaces and femoral heads
- Stem subsidence
- Leg length discrepancy
- Soft tissue tension
- Polyethylene wear (delayed onset instability)
ICJR: How common is this complication, and what can surgeons do to prevent it?
Dr. Deshmukh: Instability occurs in approximately 1% to 2% of primary THA patients. A detailed history, physical, and preoperative assessment of patient risk factors is the necessary first step, helping the surgeon develop a preoperative plan to minimize the risk of postoperative instability. During the procedure, key steps for preventing instability include:
- Meticulous soft tissue handling and repair
- Accurate component positioning
- Careful intraoperative assessment of leg length, offset, impingement, range of motion, soft tissue tension, and stability
The Ranawat combined anteversion test is helpful for assessment during a non-guided posterior approach THA. In addition, a Steinman pin placed in the infracotyloid groove is useful for intraoperative assessment of leg length. The surgeon can use various intraoperative landmarks and navigation tools to assist in accurately placing components. It is prudent to use large-diameter femoral heads and dual-mobility components in patients at high risk for instability.
ICJR: How do you work up the patient who presents with a suspected dislocation to determine the source of the instability?
Dr. Deshmukh: It’s important for the surgeon to take a detailed history and conduct a careful clinical exam to understand the mechanism of dislocations. The exam should include assessment of the patient’s gait, leg length, and motor strength, especially strength of the abductors, as well as the position of the lower extremity at rest.
The surgeon should obtain radiographs, including a low anteroposterior (AP) pelvis, AP and cross-table lateral of the affected hip, and lumbar spine. A CT scan can be a very important diagnostic tool as it can help the surgeon determine orientation of the acetabular and femoral components and the presence of any retained osteophytes. If adverse local soft tissue reaction (ALTR) or abductor incompetence is suspected, a MARS MRI can help diagnose the extent of damage to the soft tissue envelope.
Serum cobalt and chromium levels will need to be checked if ALTR is suspected. Adverse local soft tissue reaction may result from the use of metal-on metal bearing surfaces or from trunnionosis. An infection work-up should also be performed.
A review of the operative report, if available, will give the surgeon information on the surgical approach, any intraoperative issues, and the type of components used. In general, instability follows the direction of the surgical approach used.
ICJR: What are your indications for a revision THA in these patients?
Dr. Deshmukh: Sometimes, a dislocation is an isolated event and the hip will remain stable after closed reduction. Any patient presenting with recurrent instability – in other words, more than 2 or 3 episodes of instability – needs to be worked up and considered for a revision. In the presence of modular, well-fixed, and well-oriented components, subtle adjustment of soft tissue tension with bearing surface exchange may help. This includes adjustment of leg length and offset and placement of hooded liners and large diameter femoral heads or dual-mobility components. Component-to-component or component-to-bone impingement needs to be recognized and addressed during a revision.
A revision procedure is indicated if instability is caused by:
- Gross or subtle malorientation of the components
- Bearing surface wear
- Infection (2-stage revision required)
If closed reduction is attempted but does not resolve recurrent dislocation, open reduction via a revision will be necessary.
Ajit Deshmukh, MD, is a Clinical Associate Professor of Orthopedic Surgery in the Division of Adult Reconstruction at NYU Langone Orthopedic Hospital, New York, New York.
Disclosures: The author has no disclosures relevant to this article.