الفهرس 
AcknowledgmentOnly 14 pages are availabe for public view
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Abstract
SUMMARY
Instability following total hip arthroplasty (THA) is a significant and unfortunately common complication, Most dislocations occur within the first 3 mo following surgery. The cumulative risk of dislocation does not remain constant following THA, increasing with time due to trauma, polyethylene wear, increased pseudocapsule laxity and deteriorating muscle strength.
Well understanding the anatomy of hip joint (anatomy and internal geometry of proximal femur and acetabulum) and biomechanics of hip joint (biomechanics of normal hip, pathologic biomechanics, and biomechanics of THA) is a critical point for practice in hip arthroplasty.
The risk of dislocation is influenced by patient factors such as age, sex, previous surgery, and cognitive or neurologic disorders as well as surgical factors, including surgical approach, choice of implant, soft tissue and surgeon experience.
The risk of occurrence of dislocation is also influenced directly or indirectly by factors related to design and implantation procedure such as range of motion, impingement implant fixation, and tissue damage during implantation and tissue tension after THA and component orientation (cup, stem).
Regarding to the cementless implant Hereinafter are illustrated the principal factors that contribute to the outcome: stem material ,stem design ,cup insert material , stem superficial texture, coating , load and load peaks ,and quality of surrounding bone.
The first step in management of unstable THA is the accurate evaluation which should follow a standardized and methodical approach to allow formulation of the ideal treatment plan. The evaluation include the History: How the dislocation occurred, an inquiry into other potential previous episodes of instability or dislocation, Questions regarding the presence of infectious symptoms and a review of previous documentation, including operative notes.
And physical examination: Assessment of both lower extremities and additionally, if clinically indicated, diagnostic tests (White blood cell count, -Erythrocyte sedimentation rate, -C-reactive protein level).
Evaluation also include radiographic evaluation: The femoral component should be assessed for (the version of the femoral component, Head-to-neck ratio, Loosening, Neck length and offset, Angulation and Any evidence of subsidence), and acetabular component assessed for orientation.
The initial management of an unstable or dislocated THA is closed reduction. If the patient has an acceptable stable range of motion and post-reduction radiographs demonstrate acceptable alignment and position of components. Then treatment with external bracing often results in good outcomes.
Surgical revision should be a strong consideration in patients who have a secondary dislocation, who have failed conservative management, or obvious component malalignment is noted on plain radiographs. Surgical options include revision of components; exchange of modular components such as the femoral head and acetabular liner; bipolar arthroplasty; tripolar arthroplasty; Use of a larger femoral head; use of a constrained liner; soft tissue reinforcement and advancement of the greater trochanter.
Modular component exchange is typically reserved for patients who do not have gross malpositioning of any components or, significant abductor weakness; Constrained liners particularly useful for the surgical management of recurrent dislocation in the setting of: abductor deficiency, recurrent dislocation of undetermined etiology and in patients with multiple dislocations due to neurological impairment.