Most hip dislocations occur within the first 3 months after surgery. Risk factors for dislocation include surgical approach, soft tissue tension, component malposition, and femoral head size. Treatment depends on the cause but may include closed reduction, bracing, modular components, larger heads, constrained liners, or soft tissue reinforcement. Surgeon experience also impacts dislocation rates.

1. INSTABILITY AFTER TOTAL
HIP ARTHROPLASTY
Dr. Giridhar Boyapati
M.S (Ortho), FIJR
MAXCURE HOSPITALS
HITECH CITY, HYDERABAD

2. Instability is one of the most common complications of THA
Dislocation rate of :
Primary THR : 3.2%
Revision THR : 7.4%

3. Most of dislocations occur within the first 3 months following
surgery
50%-70% of dislocations occur within the first 5 week to 3 months
postoperatively, and more than 3/4 th of dislocations occur within
the first year 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
The cumulative risk of dislocation within the first postoperative
month is 1% and within the first year approximately 2% . Thereafter,
the cumulative risk continuously increases by approximately 1% per
5-year period and amounts to approximately 7% after 25 years

4. Late dislocations are caused by progressive improvement in motion after surgery
Late dislocations are more likely to be come recurrent and require surgical intervention

5. ● Cranial dislocation
– Excessive inclination of the cup, abductor insufficiency, polyethylene wear
– Dislocation along with adduction of the extended hip joint
● Dorsal dislocation
– Insufficient anteversion or retroversion of the cup, joint hyperlaxity, primary or
secondary impingement
– Dislocation with internal rotation and adduction of the flexed hip joint or with deep
flexion
● Anterior dislocation
– Excessive combined antetorsion of stem and cup, joint hyperlaxity, primary or
secondary impingement
– External rotation and adduction of the extended hip joint.

7. Positional dislocations had a reoperative rate of 17%
Soft tissue imbalance had a reoperative rate of 46%, and 15% of
these hips ultimately underwent resection arthroplasty.
Component malposition had a reoperative rate of 77%, and in 33% of
those hips the final result was a resection arthroplasty.

8. RISK FACTORS
ANATOMICAL FACTORS
EPIDEMILOGICAL FACTORS
SURGICAL FACTORS

9. ANATOMICAL FACTORS
TROCHANTERIC NONUNION
ABDUCTOR MUSCLE WEAKNESS
INCRESED PREOPERATIVE ROM

10. EPIDEMILOGICAL FACTORS
FEMALE GENDER
AGE > 80
NEUROMUSCULAR DISORDERS
COGNITIVE DISORDERS
ALCOHOLISM
PREVIOUS HIP SURGERY
PRIOR HIP FRACTURE
PREOPERATIVE DIAGNOSIS OF OSTEONECROSIS
INFLAMMATORY ARTHRITIS

11. SURGICAL FACTORS
Surgical approach
Capsular repair
Soft tissue tension
Component malpositioning
Femoral head size
Impingement
Surgeon experience

12. SURGICAL APPROACH
Dislocation rates of :
1.27% for Trans-Trochanteric,
3.23% for posterior (2.03% with capsular repair),
2.18% for anterolateral
0.55% for the direct lateral approach

13. 75% to 90% of dislocations are in the posterior direction, thus
surgical approaches that compromise posterior soft tissues
theoretically could contribute to posterior instability
Therefore, when risk of dislocation is of particular concern, the
posterior approach historically has been the least favoured.
With adequate soft tissue repair there is ten fold reduction in
dislocation rates ( 4.46% to 0.4%)

14. SOFT TISSUE TENSION
Soft tissue tension, influenced by the
Joint capsule
Short external rotators and gluteal muscles
Femoral offset
Reconstruction of the posterior capsule
and short external rotators and restoration of femoral offset has
been shown to significantly reduce dislocation rate

15. FEMORAL HEAD SIZE
The larger the femoral head,
the further it must sublux
before it can dislocate, a
distance referred to as the
Jump distance.
Berry et al[29] in a study of
21047 THAs, found a
significantly decreased rate of
dislocation with the use of
larger femoral heads in all
surgical approaches.

17. Modular femoral head components that have extension or
SKIRT to provide additional neck length :
Reduce head to neck diameter
Reduce ROM
Reduce Stability

18. COMPONENT MALPOSITION
Component malposition is the most common cause of instability following
THA.
Excessive anteversion of the acetabulum may result in anterior dislocation
Excessive retroversion may result in posterior dislocation
Excessive inclination of the cup can lead to superior dislocation with
abduction
If cup is inclined almost horizontally, impingement can occur in flexion
and hip dislocates posteriorly.

19. Ali Khan et al found that the most common surgical error was placement of the acetabular
component in excessive anteversion and abduction
Forward rotation of the pelvis must be taken into account, or excessive retroversion of the cup
can result
In lateral position :
Women with broad hips and narrow shoulders there is a tendency to implant the cup more
horizontally
Men with narrow pelvis and broad shoulders there is tendency to implant the cup more vertically
Placement of acetabular component in orientation relative to the operating table produces
inadvertent retroversion relative to pelvis

20. SAFE RANGE
ANTEVERSION OF 15° ± 10°
INCLINATION OF 40° ± 10°
Dislocation rate with cup in safe range is 1.5%, whereas
6.1% of those outside this safe range
Lowest risk values for dislocation were 15deg anteversion
and 45 deg of inclination

21. COMBINED ANTEVERSION
Sum of ante version of cup and stem
Total of 35 deg
Acceptable range 25 to 50
Dorr proposed the so-called “stem-first” procedure for the
CA technique in which the stem is set first, and cup
alignment is determined to consider the target CA value.

22. RANAWAT TEST FOR CA
CA can be easily checked intraoperatively by internally rotating the
femur until the neck of the prosthesis is perpendicular to the opening
plane of the cup.

23. INTERNAL
ROTATION TEST
To assess the capsular
integrity and risk of
hip dislocation after
the posterior
approach

24. A positive internal rotation test is indicative of proper healing of the
posterior soft tissue, which includes the capsule and short external
rotators, with a specificity of 100%.
The high sensitivity and specificity of the internal rotation test for the
healed capsule/tendon unit makes it a clinically useful test during
the physical examination to demonstrate an appropriate repair and
thus safely discontinue hip precautions, thereby allowing patients to
return to full activities of daily living.

25. IMPINGEMENT
Bone or cement protruding beyond the flat surface of the
cup; it serves as a fulcrum to dislocate the hip in the
direction opposite to its location.
Residual osteophytes
Capsular scar tissue
Heterotopic ossification
Impingement of femoral neck on liner elevation

27. Surgeon experience
The rate of dislocation has an inverse relationship to the experience
of the surgeon.
For every ten primary arthroplasties performed yearly, there is a
50% reduction in the postoperative dislocation rate

28. MANAGEMENT
CLOSED REDUCTION
BRACING
MODULAR COMPONENT EXCHANGE
BIPOLAR AND TRIPOLAR ARTHROPLASTY
LARGE FEMORAL HEAD
CONSTRAINED LINER
SOFT TISSUE REINFORCEMENT
GREATER TROCHANTER ADVANCEMENT

29. CLOSED REDUCTION
If the components are in satisfactory position, closed
reduction is followed by a period of bed rest.
Abduction Orthosis : maintain hip in 20 deg abduction and
prevent flexion more than 60 deg
immobilisation for 6 weeks to 3 months is recommended

30. MODULAR COMPONENT EXCHANGE
Increasing femoral head size
Increasing neck length
Various liner options.
Minor malposition of a acetabular component can be managed with changing the position
of the liner or adding an elevated rim liner
Inadequate femoral neck length require exchange of modular head or revision of femoral
component.
MALPOSITION OF MORE THAN 10 DEGREES REQUIRE REVISION OF THE
COMPONENTS

31. Bipolar and tripolar arthroplasty
The bipolar arthroplasty component consists of a small femoral
head located inside a polyethylene shell that is then covered by a
larger femoral head. This theoretically allows motion between the
small femoral head and the liner as well as the larger femoral head
and the acetabulum.
Placement of a bipolar prosthesis inside an acetabular component
with a liner is known as a tripolar arthroplasty

33. LARGE FEMORAL HEADS
Larger femoral heads (e.g. 36 mm) allow a wider mechanical range
of motion compared with smaller head diameters (e.g. 28 mm)
before the neck of the prosthesis strikes the rim of the acetabular
component .
In addition, the distance a larger femoral head has to move away
from the center of the acetabular component (“jumping distance“)
before it can dislocate over the rim of the cup is longer. Thus, a
larger head diameter offers better protection against dislocation

34. Kung et al evaluated 230 patients for the effect of femoral head size
(28 mm vs 36 mm) on postoperative stability.
At a mean follow up of 27 mo, they found that the use of the larger
femoral head brought the dislocation rate from 12.7% down to 0%.
However, if the abductor mechanism was absent, there was no
statistically significant reduction in dislocation rate.

35. Disadvantages:
Inlay thickness has to decrease with increasing head diameters
Increased abrasion along the head-neck plug connection
The stabilizing effect is lost in case of abductor insufficiency
Increased range of motion promotes secondary impingement with
resulting contact between proximal femur and pelvic bone.
For these reasons, femoral heads with diameters of more than 36 mm are
not normally used.

36. Constrained liners
Constrained liners are designed to physically resist dislocation of the
femoral head by locking the head into the acetabular cup
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
liners offer the ability provide enhanced stability to a hip without the
need to revise well-fixed, well-positioned acetabular components

38. Soft tissue reinforcement
Additional static restraint to augment the deficient posterior capsule or
enhance a deficient abductor mechanism
Lavigne et al were the first to report the use of an Achilles tendon
allograft placed between the greater trochanter and the ischium to
reduce the range of internal rotation and enhance stability.
Barbosa et al described the successful use of a synthetic ligament
prosthesis to treat patients with recurrent posterior dislocation of THA
Soft-tissue procedures provide an additional approach to achieving
THA stability in patients who are poor candidates for other options
such as constrained liners.

39. Greater trochanter advancementIncrease abductor tension and stability
Improve the resting length and functioning of the abductor
mechanism, which consequently affords increased hip
stability in 81%-90% of cases.
The advent of modular implants allows the surgeon to
increase femoral neck length to accomplish the same goals
without the potential morbidity of greater trochanteric
nonunion.
Used only as an option when there is proximal migration of an
ununited trochanter after a trochanteric osteotomy

40. Resection arthroplasty
Patients who fail to respond to any measures may be
candidates for end stage revision to a resection arthroplasty.