Obtained by Vicky Marlow Freelance Patient Advocate Volunteer
Dr. Kurtz had an article online that had concerns about the head neck ratios of hip resurfacing compared to total hip replacement. We wanted to provide potential hip resurfacing patients with opinions of other top resurfacing surgeons to explain their views.
Dr. Bose Reply
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-op 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.Dr. Vijay C. Bose
Consultant orthopedic surgeon Chennai, India
Dr. Su Reply
Very nice to hear from you. I’m sorry for the delay in responding, but I wanted to take time to craft a thoughtful response on this subject, as I’m sure many people look to you for sound advice.
I have read over the material at this website many times. I have concluded, as I’m sure you have, that it was written by someone who has a very limited experience with resurfacing. In short, it full of speculation and non-factual conclusions, interspersed with a few truths. As such, it can appear convincing at first glance, but when really scrutinized, many of the arguments don’t hold water.
In the section entitled the biomechanics of hip resurfacing, it is actually true that a resurfacing implant will have a poorer head neck ratio than a comparable large diameter metal on metal THR. It is also true that head-neck ratio influences that amount of motion prior to impingement. However, the question is whether this would translate to a clinical difference or not. Furthermore, some of the arguments that he makes are not accurate:
“If a small lady has a 46 mm head diameter, her hip resurfacing would likely have a 46 mm head diameter, a 40 mm neck diameter, and a 1.15 (46/40) head-neck ratio. If a large man has a 58 head diameter, his resurfacing head would likely have a 58 mm head diameter, a 49 mm neck diameter, and a 1.18 head-neck ratio.”
First off, this argument doesn’t account for the acetabular size at all. As we know, the acetabular and femoral head sizes are linked together. So the fit of the acetabulum will influence what head size is used. So, it isn’t a rote fact that the neck will be 6-9 mm smaller than the head diameter. The relationship of the acetabulum and the femoral head size is such that the head neck ratio doesn’t change much from preop to postop. I have performed research analyses on the typical head ratio following resurfacing. It is approximately 1.34, as compared to 1.39 preoperative. The reason that it is slightly lower is to preserve acetabular bone.
“One additional factor influences the head neck ratio in hip resurfacing, the femoral component height. Sometimes, a surgeon will attempt to lengthen a patient’s leg during hip resurfacing by removing less bone off the top of the femur and placing the femoral component higher or more proud. When the surgeon raises the femoral component, he/she inadvertently raises the femoral head/neck junction. The femoral neck diameter increases as you move up the femoral neck until it fads into the femoral head. Therefore, if a surgeon raises the femoral component, he/she is ensuring an increased neck diameter and a sub-optimal head/neck ratio.”
This is a dangerous technique, one that is not recommended by any experienced resurfacer. By leaving the femoral component higher, you will increase the risk of fracture.
“Second, the acetabular bone has a particular shape to the anterior wall that allows more motion before the femoral neck hits the acetabular rim. “
This doesn’t change, even after the resurfacing. If put in properly, the acetabular component should still be below the native acetabular rim.
“In a hip resurfacing, the femoral neck impinges of the metal rim of the acetabular component. I feel that the repetitive hard impact is the main cause of femoral neck fractures. “
The last statement is ridiculous and unfounded. This is certainly not the main cause of fracture, otherwise all the fractures would occur late, and not within the first few months.
In any case, the argument about a poorer head neck ratio is a theoretical and don’t have clinical relevance. That is, they don’t translate to any detriment to patients. It clearly can’t mean that motion is going to be limited, otherwise how do we see so many success with ROM, such as yourself? It is true that the components have to be put in perfectly to ensure good ROM, and I will test this while the patient is on the table, so that I may adjust it if necessary.
I hope that is helpful.
Regards Dr. Edwin Su
Dr. Rogerson Reply
I read over the discussion of the head-neck ratios and impingement that you forwarded which was very thorough. There is no question that the head to neck ratios of a total hip arthroplasty with a small diameter neck are larger than with resurfacing arthroplasty, especially a metal on metal big femoral head construct. This would translate into less impingement and greater stability which increases as the head size increases. If biomechanics were the only factor involved in the choice of a prosthesis then I would opt for the MoM Big Femoral head type every time. However, there are many other factors to consider including the technique used in the procedure.As you can see from the discussion even small traditional heads with small necks have a better head-neck ratio than resurfacing and yet in clinical practice traditional hips have a dislocation rate in most series of 3-4% in primary replacement which increases up to about 20% in revisions. This compares with a dislocation rate around 0.3-0.4% in resurfacing series. Factors that explain this dichotomy are the more natural soft tissue balance and more accurate leg length and offset that are associated with resurfacing and the decreased “jump distance” of a small head prosthesis when compared to large head prostheses (the actual distance the head has to move before it dislocates is much less for a small head than a large even though the head-neck ratios may be greater in the small head). In our series and especially in McMinn’s and Treacy’s 10 year series, late neck fracture has not been a clinical problem so if impingement with a resurfacing was such a detriment we would expect to see 1) impingement pain, 2) decreased range of motion, or 3) late neck fracture. This does not appear to be the case if care is taken technically to not notch the superior neck of the femur during the procedure. Of the late neck fractures reported, the biggest factor seems to be avascular necrosis rather than impingement. Obviously, this is very rare also since the 10 year survivorship for osteoarthritis is 99.6%! In summary, I agree with the statements presented regarding head-neck ratio and feel it particularly supports using a large head vs. a small head traditional prosthesis. However, clinical results and other biomechanical factors would seem minimize its importance as regards to resurfacing arthroplasty as noted above. The other major advantages of resurfacing in a young, active patient population ( bone sparing, high activity friendly, diminished proximal femur osteopenia, and ease of later revision) far outweigh the theortical considerations presented. Thanks for the opportunity to respond. Sincerely, John Rogerson, MD