Head Neck Ratio in Hip Resurfacing by Dr. Vijay Bose
Thanks for the mail. I read Dr. Kurtz thoughts on hip resurfacing in his website. His concerns are very valid but I cannot agree with his conclusions.
In short, his concerns only underline the fact that bad results of resurfacing are due to badly done resurfacings. The head neck ratio is an important determinant of range of movement and prevention of impingement. In a patient with normal anatomy, if one is careful to restore anatomy the range will be like pre-0p range of movement before the onset of arthritis. This is a simple concept.
However many patients especially young osteoarthritis will have FAI ( Femoro - Acetabular impingement) as the source of their arthritis. It is of paramount importance to recognize it and deal with it time of surgery. Again patients with an mild unrecognized slip in their earlier years will have OA in the later years. Here again it is crucial to recognize and deal with it at the time of surgery. As the head component in a resurfacing is centered on the neck and not the head, correct placement will restore the head neck offset to a large degree. During the surgery the metal cap will look very eccentric on the head.
Surgeons with less experience in resurfacing will think this is wrong and will just put a cap on the translocated head resulting in very low head neck ratio which will lead to problems postop.
In some severe cases, even if done correctly there may not be adequate head neck offset. This is very rare and in this instance one has two choices. In a very young patient, I would trim the ant neck to re-create the offset. In an older patient I would proceed to use a stemmed component with the same acetabular cup. One cannot underestimate the importance of bone conservation in a young patient.
In a patient whose head - neck offset is carefully restored to 'normal ' during surgery and the acetabulum inserted in correct orientation, patient will have 'normal' movement postop. Only a contortionist will need more than 'normal' movement. Although in theory a large head THR can have supra normal movement, this never happens in clinical situations because apart from the head neck ratio there are many other factors determining ROM like muscle tension etc.
By stating 69 degrees as the functional ROM In resurfacing, is Dr. Kurtz suggesting that resurfacing patients will not be able to sit in a chair as that would require 90 degrees?
The mathematical calculations is very different from actual clinical results in the human body. The most practical example of this is in India where most patients would sit on the floor even if the surgeon advises them not to as it is a very important social requirement.
We did a study in our unit and found that 20 % of conventional THR were able to sit and 76% of resurfacing patients were able to sit. This again reiterates the importance of surgical technique.
Purely by choosing a particular prosthesis one cannot guarantee a near normal ROM- it has to be installed correctly. However the resurfacing/ anatomical head is the best tool in the surgeon's hands to restore near normal ROM.
Dr. Kurtz also has mentioned component height which would give a prominent head neck junction if not seated. I fully agree with this and it would cause serious problems if not seated. The bottom line is again technique related and one must fully seat the component.
The next issue is impingement which he has raised. The concern in very valid because resurfacing acetabular components typically subtend a larger angle at the periphery than conventional THR cups. Therefore it is more difficult to bury the anterior edge beyond the bone margin in a resurfacing. I would do this in all cases and would never accept ant edge of the cup to be more proud than the bony margin. Therefore the issue of neck- prosthetic impingement does not arise in my opinion. Again is a matter of surgical technique.
Some of his statements, are simply not true—like the ones given below.
One does not remove more acetabular bone in the acetabulam than in a THR. - if someone is doing this he is doing something seriously wrong. I have explained this concept earlier. If any resurfacing surgeon is doing this he must be condemned. The incision for resurfacing is not bigger than for THR. It has been published by Derek McMinn that Hip resurfacing can be done by MIS and results are same.
My incisions for both resurfacing and THR is about 10 to 14 cms and the length variability depends on the constitution of the patient and not on the procedure. If a surgeon is using larger incision for resurfacing than for THR, it is not wrong but is in the learning curve of the procedure. Arguments like that of the removal of labrum and cutting of the capsule in a resurfacing will cause problems sounds to be weak attempts to pick holes in the outstanding functional results that have so far been achieved in the last 12 yrs in resurfacing. The capsule is not removed in a resurfacing but carefully preserved and stitched back capsule to capsule ( the NCP approach or the neck capsule preserving approach for resurfacing). It is certainly true that the surgeon has to give much more importance to the preservation of neck capsule in resurfacing than in a THR.
It appears to me surgeons confuse many aspects of resurfacing. The old poly resurfacings results must not be mixed with the modern metal on metal resurfacings.
There are two dif concept in a resurfacing which was introduced to the orthopedic community at the same time and hence gets mixed up. The first is the use of an anatomical sized bearing. This implies the head diameter to be the same as that of the native head. It is important to understand that the aim is not to put in the biggest sized head that is possible. If a larger than a native size is uses, it will bring a dif. set of problems. Anatomical sized bearing can be done with a resurfacing or with anatomical metal on metal THR ( people refer to this wrongly as large head—it is actually the correct head and all other heads are indeed small heads). Now, currently one can use the BMHR as well. I have attached the pics which illustrates it. Hip Resurfacing is not the aim here - the goal is to restore an anatomical bearing which would be best attempt at restoring near normal function. One has to use the best devise to achieve this goal.
Restoring an anatomical bearing is the goal in a high value hip—high value hip means in patient who have a lot of demand out of their hips. An elderly sedentary patient can have any hip and any articulation. It would make no difference. However an wear resistant anatomical bearing is the goal in a patient who has demand of the hip for occupational, recreational or social customs.
This is the first aim. The next issue is of bone conservation. Importance of bone conservation is determined by relative importance of 3 factors, namely the age, the activity level and the bone stock. Bone preservation is not a static concept. Bone conservation would be of immeasurable value in a 25 yrs old and would be probably be a contraindicated in 80 yrs old due to the risk of femoral neck fracture. I have attached a pic to illustrate this point.
Thus there are two dif issues here - the use of an anatomical sized bearing & bone conservation. These are independent issues. As both these concepts came simultaneously with the advent of resurfacing there has been a hotch-potch with many confusing these two.