Sir,
I thank Drs.Kini and Maduri for their interest in our article.
Von Knoch et,al.(1) detailed several factors that make hip
arthroplasties
vulnerable to late dislocation. These include longstanding problems such as component malposition and recurrent subluxation, and new onset
problems such as trauma, polyethylene wear, and neurologic changes.
These factors considered individually or in combination contribute to
the
clinical picture of instability or stability. The use of anatomic dimension
femoral
heads with minute wear rates theoretically should alter this balance in
the
favorable direction of stability. Clearly any estimation of the influence
of this
alteration over time can only be speculative. Still, the nature of this
response
invites speculation. I would suggest that for dislocation to occur early
or late
there must be a failure of the soft tissue restraints. Absent competent
hip
ligaments, a total hip will always be vulnerable to dislocation. A greater
head/neck ratio increases the range of motion to dislocation, but does not
prevent it.
Socket malposition leads to early catastrophic failure for implants
with hard
on hard bearings. When these implants are well positioned, they do not
seem
subject to extensive early wear. Late dislocation because of gross or
cumulative wear remains to be demonstrated over time.
Recurrent subluxation like dislocation may be multifactorial, but
clearly
cannot occur if there are adequate soft tissue stabilizing elements.
Trauma
may lead to soft tissue failure or loss of fixation with ensuing
malposition.
Neurologic changes compromise dynamic stability.
Given the above, I would suggest that the factors leading to late
dislocation
must perforce include impingement, but impingement alone does not cause
dislocation. The greater the impingement free range of motion and enhanced
jumping distance that larger femoral heads offer may lower the chances of
any particular hip motion leading to dislocation, but absent soft tissue
constraint, at some point dislocation will still occur.
The field of hip arthroplasty is replete with brilliant technology
that did not
stand the test over the long term. I agree follow up over time is
paramount.
We did not perform serum and urine metal ion studies because we deemed that they were not
necessary given the attention this subject has received in the literature.
The patients with dysplasia in this study had Crowe I deformities
and were
not a challenge for pure pressfit fixation.(2) Amstutz et al have reported
on
hip resurfacing in Crowe I and II deformities.(3) Since this report, I
have
performed this surgery in Crowe II deformity but in my experience, the
adjuvant fixation offered by a dysplasia socket was helpful to secure the
cup.
I cannot comment on this type of implant in Crowe III or IV
problems, but
suggest such surgery is technically feasible as evidenced by the
reports of hip resurfacing in patients with advanced dysplasia. (4) Again
I
would speculate that the conceptual initial stability benefits this class
of
implant offers would be advantageous in these cases that are notable for
early dislocation.
No objective quantitative range of motion study was done. In spite of
the
enhanced in vitro range of motion that an increased head/neck ratio
offers,
in vivo results may not correspond. In this study, the post operative
dislocation precautions included limitation of hip flexion for six weeks.
This
proscription may lead to soft tissue restriction of motion no matter the
head/neck ratio.
References:
1. Von Knoch M,Berry DJ,Harmsen S etal:Late dislocation after total
hip
arthroplasty.J Bone Joint Surg 84A:1949,2002
2. Crowe JF. Mani VJ, Ranawat CR. Total hip replacement in congenital
dislocation and dysplasia of the hip. J Bone Joint Surg(A) 1979;61-A:15-
23.
3. Amstutz HC, Le Duff MJ, Harvey N, Hoberg M. Improved survivorship
of
hybrid metal-on-metal hipresurfacing with second generation techniques for
Crowe-I and Crowe-II Developmental dysplasia of the hip. L Bone Joint Surg
(A) 2008;90(Supplement_3):12-20.
4. McMinn DJW, Daniel J, Ziaee H, Pradhan C. Results of the
Birmingham hip
resurfacing dysplasia component in severe acetabular insufficiency. J Bone Joint Surg(Br) 2008; 90-B:715-23.
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