Dislocation of Total Hip Replacements -

Definitions

Usual direction of dislocation is posterior – 60-90%
- These occur by placing the leg in flexion, adduction & internal rotation.

This involves 4 major areas
- Component design
- Component alignment
- Soft tissue tensioning
- Soft tissue functioning

primary arc range
- is the amount of arc possible in the ball & cup articulation before there is impingement (primary impingement) & levering out. The major determinant of this is the head neck ratio. A larger head neck ratio allows further arc of motion before the head levers out
- Head
  - primary arc range in a 22mm head with a standard taper is 100°
  - with a 28 mm head the primary arc is 120°
- Neck
  - Narrower necks also provide more stability by increasing the primary arc range
- Liners
  - Elevated liners ↓ primary arc range
  - A constrained liner will dramatically ↓ primary arc range
excursion distance
- This is the distance the head has to be levered out before dislocating, once it starts to impinge
- In a 32mm head this is 16mm
- in a 22mm head it is only 11mm

goal of component alignment is to centre the patient’s primary arc range in the middle of the patient’s functional range of motion
If the components are malaligned, the articulation will be very stable in one direction, but in the other will consistently reach the point of primary impingement in normal day to day functional positions
desired cup position
- 35-45° coronal tilt
- 10-25° anteversion

key to correct soft tissue tensioning is the abductor complex
To obtain correct tension in the abductors, the offset & neck lengths must be reconstituted
A short neck length will not only slacken the abductors, it will cause trochanteric impingement, with abduction causing the hip to lever out

Coordinated muscle action is necessary to maintain a stable hip, & can be affected by a problem anywhere along the neurological axis.

Note that there is no statistical /clinical evidence to confirm that there is a higher dislocation rate with posterior approaches or smaller diameter femoral heads
32mm heads theoretically have a lower rate of dislocation (but higher volumetric wear rates)
In patients with larger head, impingement between osseous femur & acetabulum is more common; with smaller head sizes the impingement is between liner & neck
Clinical data show that patients with larger outer cup diameters have a higher dislocation rate. Cups larger than 62mm, used in association with 28mm heads, were associated with ↑ dislocation in one study. The cup to head ratio is the important thing, & if the head size should be ↑ as the cup size is ↑

closed reduction & hip abduction brace
- successful in around 2/3
While the patient is anaesthetized, the ROM should be assessed
- If dislocation occurs only at the extremes of movement, then the prognosis is good, but if it occurs in the middle of the patient’s functional arc revision is much more likely
If there are more than 2 dislocations, should consider revision.

is successful in around 2/3 of cases if a cause is identified
If no cause is identified the success rate is lower
If the components are malaligned, they need to be revised.
Use a bigger head (32mm) if possible, as the stability it provides is more important than polyethylene thickness
If offset is inadequate it should be ↑
If the components are well aligned & the offset appears reasonable, trochanteric advancement is a good option
Conversion to a constrained liner leaves the patient with a very restricted ROM; if they do not adhere to this they will quickly lever the socket out
Another option is use of a jumbo head, in conjunction with a large cup. Problem will be with excessive wear, & the long term results with respect to osteolysis are unknown
Allograft material can be used to augment a deficiency of the posterior part of the capsule
Resection arthroplasty is used in two main situations:
- Multiply revised patient with significant loss of bone stock & soft tissue stock
- Psychiatric patient who deliberately repeatedly dislocates the joint