| Dr. Bose
I have now done about 185 resurfacings for AVN
cases over a 5 year period with many patients crossing the 4 yrs
mark. It is interesting to note there has not been a collapse or
fracture neck of femur even in a single patient.
It is wrong to think that the AVN continues forever in the
femoral head. AVN is a one time event in which a strikingly
similar sector of necrosis occurs in most femoral heads ( anteo
supero lateral part) due to blockage of presumably the same
vessel in all patients. This sets off a series of changes which
are is marked by sectoral collapse. This is primary collapse of
AVN and most patients are likely to develop it. Any kind of core
decompression / bone grafting is a surgical attempt at
preventing /postponing this event. These joint salvage
procedures ( according to literature) achieves their goal in
about 30 – 50% of cases.
The rest of the collapse (which at times is confused with
primary AVN collapse )even by medical personnel is actually
secondary mechanical collapse and this occurs because of 3
factors.
1.hip stiffness, ( more the stiffness the more the likelyhood of
secondary collapse)
2.wrong biomechanics leading to point loading.
3.soft bone ( non wt bearing and NSAID abuse).
However once resurfacing is done secondary collapse will not
continue as the normal biomechanics and range of movement is re
established . The portion that is already collapsed ( primary or
secondary) has to be taken out and substituted with cement or
bone graft at the time of surgery. This is a simplistic
explanation for people not familiar with the concept. However
this does not represent the complete story.
Please read on if you are a medical personnel.
The 3rd type of collapse that can occur is specific to
resurfacing and is called as ‘Global AVN’ tertiary collapse ,or
delayed primary failure of resurfacing. In this the resurfaced
head slowly tilts and falls off over a period of months. This is
the number one concern today in the field of hip resurfacing.
There are many theories as to why this occurs but the most
plausible one is that it is procedure induced and it involves
disturbing the soft tissues of the neck and the head-neck
junction of the femur ( not the head offemur) at the time of
surgery.
One must keep in mind that AVN occurs in individuals following
pretty trivial reasons like a fall, a single dose of steroid or
surgery in the vicinity of the hip joint like intramedullary
nailing of the femur. To assume that the varied approaches
described for resurfacing (anterior, lateral, posterior &
trochanteric osteotomy) will not cause AVN in the femoral head
is naive. It is now increasingly becoming obvious that Apical ,
sectoral primary AVN is caused during the surgical approach in a
very significant proportion of patients of any surgeon’s series
of hip resurfacings. However, this is not of any consequence and
does not compromise the result.
In summary- the primary, sectoral classical AVN occurs in a
majority of resurfacings during the surgical exposure even in
cases which did not have AVN to begin with.
However with the usage of low viscosity cement one performs a ‘capituloplasty’
on the head, similar to the vertebroplasty done in the spinal
vertabrae with the injection of cement.
This transforms the material under the resurfacing head into a
composite of live bone, dead bone and cement.
If this composite is seated on a vascular and biologically
favourable neck and head neck junction , then this composite
performs well. (The biological status of the neck and head neck
junciton is similar to health of a fracture fragment in fracture
plating surgery.ie Soft tissue cover of a bone fragment is
essential for the end arteries to supply no matter from where
the blood is coming from) However for some resion the neck
capsule and soft tissues get damaged then one gets ‘global AVN’
and the component drifts and fails. – termed as delayed primary
failure . This is independent of the fact as to whether primary
, sectoral AVN in the head was present before surgery or occured
during the time of the surgery.
Therefore , resurfacings in AVN are no different from
resurfacings done for other indications. However if secondary
collapse has been left for too long it destroys the femoral head
bone stock completely precluding hip resurfacing. If there is
sufficient bone stock at the time of surgery a AVN resurfacing
is likely to perform as well as any other resurfacing.
The 185 AVN resurfacing represents roughly half of my series of
about 400 cases.
Please feel free to write to my e-mail add bose5vijay@hotmail.com
if you have any specific queries. I would be glad to address
them.
Vijay Bose.
consultant orthopaedic surgeon
chennai |
