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 firstname.lastname@example.org if you have any specific queries. I would be glad to address them.
Fact 1 – Ortho surgeons all over the world are still not completely sure about AVN and how it progresses. We all fight with each other on this issue in our resurfacing symposia and meetings. So the jury is still out, to be honest.
Fact 2 – AVN is not always a one time event – we have seen many cases of AVN with serial MRI scans actually showing progress of AVN over a period of time. So if resurfacing were done for such patients, the AVN could continue to progress, causing persistent pain and eventual failure Fact 3 – once the AVN has led to arthritis, then the AVN itself does not progress. Further progress of damage seen on x rays is purely mechanical. Such patients can safely have a resurfacing.
In conclusion – AVN with advanced arthritis can be treated exceptionally well with resurfacing. AVN itself (in the stage where arthritis has not occured)should not be treated with resurfacing.