Dislocation


Properly reduced hip joint - ball in socket	Dislocated hip joint - ball behind socket

Dislocation of a hip replacement means the ball has come all the way out of the socket. When the hip pops out it is usually very painful and requires a trip to hospital for the ball to be replaced in the socket.

Partial displacement, called subluxation, it is difficult to diagnosis as there are other causes of "clicking and clunking" around the hip.

The risk of dislocation is highest in the early post-operative period before the soft tissues have healed. After about 6-8 weeks the muscles and fibrous tissues around the hip offer more support and resist movements that may lever the ball out of the socket. A single dislocation during this healing process does not always lead to long term problems.

However, a small number of hip replacements go on to recurrent dislocate. The risk factors and causes of recurrent dislocation have been extensively studied. For any one patient this problem is usually multi-factorial.

Risk factors and proposed causes of hip joint dislocation	Significant	Insignificant
Malposition of the cup Abduction <30° or >=50° Anteversion <=5° or >25° Medialization > High hip centre >	[Dislocations after total hip-replacement arthroplasties. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. J Bone Joint Surg [Am] 1978;60-A:217-20. Lewinnek's work was based upon only 9 dislocations from a group of 300 operations. His findings showed that anterior dislocations were fewer with anteversion 15° ±10° and abduction 40° ±10°. There was no association between cup position and posterior dislocation.]
Malposition of the stem Anteversion <15°: Retroversion > 0°: Femoral Shortening > Femoral Lengthening >
Under-correction of horizontal femoral offet Horizontal femoral Offset % of normal side Global femoral Offset % of normal side
Recuded range of motion before impingement Smaller head to neck ratio Acetabular osteophytes Femoral osteophytes Heterotopic ossification Elevated liners	[ Dislocation After Primary Total Hip Arthroplasty: Effect of Femoral Head Diameter and Operative Approach on Risk of Dislocation After Primary Hip Arthroplasty. Daniel J. Berry, Marius von Knoch, Cathy D. Schleck and William S. Harmsen. J Bone Joint Surg Am. 87:2456-2463, 2005.]	[Independent contribution of elevated-rim acetabular liner and femoral head size to the stability of total hip implants. Sultan PG, Tan V, Lai M, Garino JP. J Arthroplasty 2002;17:289-92. This work shows that for a posterior approach the added stability of an elevated rim allows more internal rotation before dislocation. It does not comment about impingement, nor is it valid when using any other approach.]
Failure of soft tissue healing After an early dislocation Poor quality tissues Trochanteric avulsion Infection - especially after hip fractures
Late soft tissue laxity Inflammatory Arthritis (Rheumatoid) Local Inflammation due to polyethylene debris Neuromuscular
Late cup wear / migration Impingement causing polyethylene wear Impingement causing ceramic fracture / chipping Loosening leading to malposition

Treatment of first time dislocation

Dislocation in the first few weeks is usually due to poor muscle tension. The x-rays of the hip joint before the dislocation and after it has been replaced should be carefully studied to exclude component malposition, implant migration and implant disassembly. A CT scan may be useful to assess cup abduction and anteversion.

If the implant appears well fixed and well positioned it is unlikely that the hip will go on to recurrent dislocation [Prognosis of dislocation after total hip arthroplasty. Joshi A, Lee CM, Markovic L, Vlatis G, Murphy JCM. J Arthroplasty 13:17, 1998 ]. A period of extra-careful rehabilitation while the soft tissues recover is all that is usually required. For patients who struggle to control the position of the leg a brace may be useful to prevent further stretching of the soft tissues.

A single early dislocation is unlikely to significantly diminish the overall outcome [Functional Outcomes Following NonRecurrent Dislocation of Primary Total Hip Arthroplasty. M.E. Forsythe, S.L. Whitehouse, J. Dick MD, R. Crawford. J Arthroplasty. 2007 Feb;22(2):227-30. http://eprints.qut.edu.au/5589/1/5589_1.pdf].

Treatment of recurrent dislocation

Recurrent dislocation has a dramatic effect, reducing confidence and in some cases patients become house bound. When a period of protected movement has failed then further surgery is recommended.

Any revision surgery requires careful planning and access to a wide range of implants and techniques. Depending upon the underlying cause of recurrent dislocation a variety of options are available to stabilise the hip. These are all major operations requiring 6-9 months of recovery.

Component Re-orientation Repositioning of the femoral stem Repositioning of the acetabular cup
Alternate articulation with high stability Large head Bipolar unconstrained Constrained articulation