Instability (INS)/Dislocation (DISL)
The incidence ranges from 0.2 to 7% for primary and 5 to 25% for revision THA [1–3].
INS is the third reason of revision THA following loosening and infection but the first reason of early revision. Greater than 50% of DISLs occur within three months postoperatively [4, 5].
The risk of recurrence following the first episode is estimated up to 33% .
- (b)Types [4–7]
- 1.Early dislocation [6, 7]
It occurs during the first 3–6 postoperative months.
It is the most common type (50–70%).
The inadequate healing and other preoperative risk factors are the main reasons.
It has a better prognosis with a lower rate of recurrence.
Usually nonoperative treatment.
- 2.Intermediate 
It occurs between the six months and five years postoperatively.
The older age, muscular laxity, and cognitive impairment are the main reasons.
- 3.Late dislocation [6, 7]
It occurs more than five years postoperatively.
It usually has a multifactorial etiology.
It has a higher rate of recurrence.
Frequently involves operative treatment.
- (c)Risk Factors
- 1.Patient’s 
- (a)Neuromuscular and cognitive disorders [5, 8]
Muscular dystrophy, dementia, alcoholism, Parkinson, MS, etc.
Muscle weakness, imbalance, and inability to comply with activity restriction
- (b)Age [9, 10]
It is not definitely considered an independent risk factor .
Muscle weakness, frailty, propensity to fall, and lack of adherence to the postoperative protocol that follows the older age are the main reasons.
Posttraumatic/prior hip surgery and history of fracture
Other underlying diseases (osteonecrosis, inflammatory arthritis, DDH) 
Patient noncompliance with activity restrictions
- 2.Surgical (not only one cause but usually multifactorial) 
- (a)Surgical approach
Although the posterior approach was considered an independent risk factor of dislocation at the past , the meticulous repair of the soft tissue and the use of large heads lessened the risk . A meta-analysis  found that the dislocation rates of posterior approach with and without repair were 0.49 and 4.46%, respectively, and 8.21 times the greater relative risk for the posterior approach without soft tissue repair than with soft tissue repair . Goldstein  et al. and Pellici  et al. reported decrease of dislocation rate with posterior capsule repair from 4.8 to 0.7% in 1515 patients and from 4.1 to 0%, respectively.
A recent meta-analysis  involving 13,203 primary THAs found dislocation rates of 1.27% for trans-trochanteric, 3.23% for posterior (2.03% with capsular repair), 2.18% for anterolateral, and 0.55% for the direct lateral approach. There were however only four prospective studies that did not have sufficient power to reach statistical significance .
- (b)Soft tissue tensioning/condition of soft tissues [4, 5]
Meticulous reconstruction of posterior joint capsule and short external rotators lessened the dislocation risk .
Non-healing of the soft tissue following revision THA increases the risk.
Trochanteric nonunion increases sixfold the risk of dislocation.
Abductor deficiency .
- (c)Restoration of offset [3, 4]
Less offset limits the soft tissue tensioning and increases dislocation risk.
Restoration of leg length 
- (e)Component positioning 
It is the most common cause of instability [5, 19].
Cup safe zone of abduction is 40 ± 10 and anteversion is 15–20 ± 5–10. It is also affected by the orientation of the pelvis and body position [4, 5, 20, 21].
Risk factors are low-volume surgeons, obesity, and minimal approaches [17, 19].
Femoral stem false positioning (inadequate offset, length, and/or version on the femoral side) is another reason.
Uncombined version of cup and stem and the effect of pelvic tilt .
X-rays more suitable for inclination than anteversion measurement. Different CT protocols have been proposed but they lack standardization .
Femoral anteversion is easier to measure on CT by evaluating the angle between the femoral neck and the axis of posterior condyles , which proved that this value differs from surgeon thought by 16.8° as a mean .
Usually femoral neck comes into contact during ROM with liner or cement, osteophyte, or heterotopic ossification creating torque and dislocation.
Medial placement of femoral component and cup is another reason.
Increased head/neck ratio decreases possibility.
- (g)Head size 
Larger head advantages: increased head/neck ratio, avoidance of a skirted component, and increased jump distance because the head sits deeper in the shell and allows greater range of subluxation before dislocation occurs [5, 16, 23, 24].
- (h)Liner profile
Neutral liners increased risk than posteriorly elevated (however posteriorly elevated increase time of neck impingement and liner wear so not uniformly recommended).
Oblique and lateralized.
- (i)Surgeon experience
Higher dislocation rate was reported among surgeons who perform < 30 THAs yearly choice of implant .
- Primary THA: first dislocation
- Patient assessment [5, 10]
History (of dislocation and surgical note)
Physical examination (LLD, ROM, neurovascular integrity)
Infection exclusion (CRP, ESR, aspiration, culture)
Imaging (AP X-rays, CT) to detect cup orientation, eccentric wear, the possibility of liner dissociation, osteophytes, bone quality and integrity, femoral offset, subsidence, angulation, anteversion (difficult) component geometry (including head-to-neck ratio), osteolysis, and component loosening
Time of dislocation is crucial for the reason/weeks or months think soft tissue tension (including muscle weakness and inadequate capsular healing and scarring), component malposition, infection, or patient noncompliance. Late dislocations (> 1 year) think stretching of the soft tissues or polyethylene wear
Direction of dislocation evaluated from X-rays and from the direction of relocation
- Closed reduction: spica
With sedation to prevent damage to implant.
In case of well-positioned and stable implant postreduction .
6–12 weeks spica (their use based on small series ).
Their use is extremely patient dependent /even the most compliant have difficulties/probably in noncompliant patient can be considered [3, 25].
Almost two-thirds have effective treatment in this way.
- Indications for operative management
Recurrent dislocation >2 times
Soft tissue tension
Malposition of components (one of the primary causes)
- Recurrent dislocation
Identify the cause to have better chance of successful management .
Assess the patient using the previously mentioned steps for primary DISL.
Revision seems to be more effective for component malpositioning, infection, and abductor insufficiency.
However, none of the available surgical procedures can uniformly solve the problem of instability .
In case of multifactorial cause, the treatment is less obvious.
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