Rationale

Postoperative dislocation is still a common, troublesome complication after THA, being the second most frequent complication¹ and the predominant indication for revision THA in the United States, representing 17–22% of all revision THAs in that country.²,³ This complication faced by the orthopedic surgeon has a high morbidity, as well as a high economic cost.⁴ Identifying patients at risk for dislocation is important, as it can help with preoperative patient education, postoperative prevention measures, and the approach in the management of primary THA instability when planning revision surgery.⁵,⁶

Clinical comment

The outcome following a first episode of THA dislocation is threefold: first, the patient will have a suboptimal clinical result; second, it will increase the risk of further episodes of instability; and finally, there will be an increased requirement for revision surgery.⁷ Therefore, the ideal solution to instability is prevention achieved through optimal surgery.⁵ Recognizing adequately the potential causative factors for instability in a patient who is going to undergo a primary THA surgery is crucial.⁸

Available literature and quality of the evidence

Quality of literature addressing appropriate investigations evaluating risk factors for instability after THA is variable with level II–III evidence. There are no randomized trials.

Findings

Due to the multifactorial etiology of hip instability, a detailed assessment of the patient and surgical plan is essential.⁶ Patient‐specific risk factors for THA instability include female gender, older age, history of previous dislocation, abductor deficiency,⁹ American Society of Anesthesiologists (ASA) score of 3 or more, hip fracture, mega or tumor prosthesis,¹⁰ multiple previous surgeries, revision THA,¹¹ altered neurologic or proprioception around the hip from neurologic or spinal disease, and, importantly, after a recent lumbar fusion surgery. There is a great deal of evidence that spinal fusion alone is a significant risk factor for instability¹²,¹³ and the most important independent predictor of dislocation within the first six months after surgery.¹⁴,¹⁵ Specific surgical precautions should be taken in this population regarding implant design and orientation, and considering DM implants and surgical approach to reduce the risk of dislocation.¹⁶,¹⁷

There is increasing concern over spinal imbalance, acetabular component position, and its relation to dislocation after THA.¹³ Restricted pelvic movement due to degenerative disc disease as well as lumbar surgery does not allow the acetabulum to open during flexion of the hip with sitting, needing high inclination and anteversion, and remaining at risk for impingement at the extremes of movement. Advances in positional preoperative imaging (such as standing, sitting, and squatting) help identify high‐risk situations and alterations consequently. Stefl et al. suggested that in patients with spinopelvic imbalance the use of DM articulation should be considered.¹⁶

The soft tissue envelope of the hip joint provides the major secondary stabilizer of the THA. Deficiency of the soft tissue envelope, especially the hip abductors, is also a well‐studied risk factor. In this manner, when utilizing the posterior approach, preservation and reattachment of the capsule and external rotators greatly reduces the risk of dislocation.¹⁸,¹⁹

The surgeon’s role in THA stability includes patient selection, choice of surgical approach and implant, technical execution, and experience. Clearly, therefore, the surgeon should understand the design implications of the diverse modular components that constitute the hip prosthesis as a whole.

Resolution of clinical scenario

• A thorough anamnesis on the characteristics of the patient is mandatory to identify the presence of possible risk factors. This should include gender, age, previous hip surgery, neuromuscular disorders, cerebral dysfunction, poor patient cognition or compliance, lumbar spine disease, and previous lumbar surgery.
  
• Patients in general should be counseled regarding the risk of THA dislocation, and patients at higher risk should be educated regarding the presence of risk factors that make them particularly high risk for this complication.
  
• The surgeon performing the surgery must identify patients at high risk and consider all options to minimize the risk of this complication.
  
• Patients with pathological spinal imbalance and a biological or surgical hip fusion that are at high risk for impingement and THA dislocation may be good candidates for increased constraint such as a DM implant.

Rationale

DM cups consist of a fixed head coupled to a mobile intermediate polyethylene (PE) liner, which articulates with a smooth metal shell. Thus, there is an inner, small diameter articulation, with a capture mechanism between the head and the liner, and a larger, unconstrained, outer articulation.²⁰ Because there is an additional bearing interface compared with fixed bearing in THA, DM hips can suffer a unique failure mechanism known as an intra‐prosthetic dislocation (IPD), in which the inner prosthetic femoral head decouples from the outer PE bearing.²⁰ IPD is irreducible by closed means and always requires surgical management and DM bearing component revision.

Clinical comment

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