Structured Hip Oral Examination Question 2
Structured Hip Oral Examination Question 2
EXAMINER: This is an anteroposterior (AP) radiograph of a 52-year-old woman who presents to your clinic with non-specific right hip pain. She had a right metal-on-metal hip resurfacing procedure performed 3 years ago. (Figure 2.4.)
CANDIDATE: The anteroposterior (AP) radiograph demonstrates a higher abduction angle (lateral opening) than normal. The current recommendations are for an acetabular abduction angle of 40. Several studies have demonstrated the importance of optimal cup positioning with regard to wear, metal ion levels and the revision rate. High cup angle has been consistently reported to lead to greater wear and higher serum metal ion levels. The head size appears small; the current recommendations are that unless a minimum 46 mm head size can be used the procedure should not be performed because of the risks of ALVAL and pseudotumours. There is no radiolucency about the metaphyseal stem, no obvious narrowing of the neck and no divot sign.
EXAMINER: What do you mean by a divot sign?1
CANDIDATE: A divot sign is a depression in the neck contour just below the junction with the femoral component often associated with a reactive exostosis. It is believe to be caused by repetitive bone-to-component abutment due to impingement.
EXAMINER: What is a pseudotumour and what is the difference between ALVAL and pseudotumour?
CANDIDATE: ALVAL (aseptic lymphocyte-dominated vasculitisassociated lesion) is caused by metal particulate debris. Patients present with localized hip pain and a localized osteolytic reaction. A more severe inflammatory reaction is termed a pseudotumour.
Several studies have described an association between pseudotumours and increased wear of retrieved components. Influencing factors include implant size and implant design (clearance and cover [arc angle]). In addition acetabular component positioning and femoral head–neck offset influence the risk of impingement and edge loading usually associated with high wear rates.2 Despite this Campbell et al. reported that in 32 THA revised due to pseudotumor several patients demonstrated minimum wear features suggesting a hypersensitivity cause.3
Therefore the origin of pseudotumours is probably multifactorial caused either by excessive wear, metal hypersensitivity, a combination of the two, or as yet an unknown cause. Pseudotumor-like reactions have also
Figure 2.4 Anteroposterior (AP) radiograph right metal-on-metal hip resurfacing implant.
been reported in non-metal-on-metal bearings. In these cases, the histological findings showed accumulations of macrophages and giant cells, again suggesting an excessive wear origin.
EXAMINER: What are the risk factors for pseudotumours?
CANDIDATE: Significant risk factors for the development of pseudotumor include female sex, age less than 40 years, small component size, hip dysplasia and specific implant designs (ASR).
EXAMINER: How are you going to investigate this patient?
CANDIDATE: A careful history and examination of the patient is required. It is crucial to determine if the pain is arising from intrinsic (indicating hip pathology) or extrinsic sources (referred pain).
Extrinsic sources would include referred pain from the spine or pelvis, peripheral vascular disease, stress fracture, tendinitis or bursitis about the hip.
Intrinsic causes include aseptic loosening, avascular necrosis, infection [Long pause].
EXAMINER: What does the British Hip Society recommend [Prompt]?
CANDIDATE: Blood cobalt and chromium ions should be measured, as these are indicators of surface wear. If levels are raised the patient will require close observation. If levels are rising and the hip is painful it may be sensible to consider revising the implant.
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(a) (b)
Figures 2.5a and 2.5b MRI of right MOM hip resurfacing implant demonstrating ALVAL mass. |
I would also order an MRI scan with metal artifact reduction sequences (MARS). This is operator dependent but can give clear images of fluid collections or solid lesions (pseudotumours) around the hip.
EXAMINER: This is the MRI scan obtained. What does it show?
(Figure 2.5.)
CANDIDATE: The MRI is a T2-weighted image coronal view, which demonstrates an intra-pelvic mass.
EXAMINER: This was a pseudotumour. In fact the mass could be felt clinically when examining the abdomen.
EXAMINER: What are you going to do?
CANDIDATE: This patient requires urgent revision surgery to the hip.
EXAMINER: She is very scared of surgery and would prefer to avoid it.
CANDIDATE: I would stress the important of early revision surgery as the longer the MOM resurfacing implant is left in place the more extensive the soft tissue destruction will most likely be.
EXAMINER: What are the principles of surgery for pseudotumours?
CANDIDATE: The pseudotumour needs to be managed with aggressive debridement of all involved soft tissue. It is important to do a thorough debridement of the abnormal tissue similar to the treatment of infection. The surgery should be preformed by an experienced hip surgeon.
Although she is still relatively young I would use a metalon-polyethylene bearing surface. A ceramic bearing surface has the potential for catastrophic fracture. We are already revising for a rare complication and we don’t want anything to go wrong again. However I would use an uncemented implant. I would keep the option of using a constrained cup open as the soft tissues may be so poorly compromised that the hip is unstable but obviously would prefer to avoid this, as components will loosen early in this situation.
It would be sensible to get a second opinion from an experienced hip surgeon as per British Hip Society guidelines to confirm and support the appropriateness of the management plan.
EXAMINER: Why bother with MOM hip resurfacing procedures? The old Charnley cemented hip replacement with trochanteric osteotomy works equally well with excellent long-term results reported from the surgeons at Wrightington.
CANDIDATE: Advantages of MOM hip resurfacings include better restoration of hip biomechanics, improved proprioceptive feedback, improved wear characteristics with no PE-induced osteolysis, increased levels of postsurgical activity, greater range of movement, reduced risk of dislocation, improved femoral bone stock mass because the neck and most of the head are retained and ease of conversion to a THA if the implant should fail.
EXAMINER: What are the contraindications for resurfacing?
CANDIDATE: These include severe osteoporosis, insufficient bone stock in the femoral head, large cysts at the femoral neck or head, a narrow femoral neck, notching of the femoral neck and severe obesity (BMI > 35 kg/m2).
Other contraindications include a history of chronic renal disease, metal hypersensitivity, those with anatomical abnormalities in the acetabulum or proximal femur and certainly caution in women of childbearing age.
EXAMINER: Is resurfacing contraindicated in women of childbearing age?
Figure 2.6 Anteroposterior (AP) radiograph of revised hip demonstrating uncemented THA with screw fixation into acetabulum.
CANDIDATE: No, although a recent annotation by De Smert in the JBJS British Edition reported that two-thirds of surgeons would exclude patients of childbearing age. Most surgeons (89%) believed that women should not be excluded.4
EXAMINER: I think most hip surgeons would now avoid a resurfacing procedure in a female regardless of whether they were of childbearing age or not.5
These are her postoperative radiographs. We kept her nonweightbearing for 6 weeks as there was quite an extensive anterior wall defect in the acetabulum but she has done very well. The hip pain has settled and the abdominal mass resolved. We were very lucky as the extensive soft tissue destruction that sometimes can be seem with this condition was absent. (Figure 2.6.)
EXAMINER: What are the outcomes of hip resurfacing compared with conventional THA?
CANDIDATE: Several recent studies report identical Harris hip scores but a greater percentage of patients with resurfacing involved in high demand activities. There is a higher revision rate in hip resurfacing compared with conventional THA.6
EXAMINER: What factors are associated with higher revision rates for hip resurfacing procedures?
CANDIDATE: These would include AVN, hip dysplasia, female sex, inflammatory arthritis, increased age, a small femoral implant and specific implant designs.
Endnotes
1. Occasionally if an examiner doesn’t know what a candidate is discussing they will enquire further. Equally the examiner may let it pass so as not to reveal their own knowledge gap. Skilful, wily candidates may be able to bait and tempt the examiner into asking for clarification so as to then appear very studious and knowledgeable. Be careful however as there is a very real danger you may irritate the examiners by coming across as a ‘know it all’.
2. An indirect way of letting the examiners know that you have read the various guidelines.
3. Campbell P, Ebramzadeh E, Nelson S et al. Histological features of pseudotumor-like tissues from metal-on-metal hips. Clin Orthop Relat Res 2010;468:2321–2327.
4. De Smet K, Campbell PA, Gill HS. Metal-on-metal hip resurfacing: a consensus from the Advanced Hip Resurfacing Course, Ghent, June 2009. J Bone Joint Surg Br 2010;92:335–336.
5. MOM hip resurfacing implants are being used much less now than previously. Whether this is an over-reaction to the ASR or not time will tell. However, from the exams perspective be very careful with what you are going to say or recommend to the examiners. Know the current guidelines and literature! Large MOM Jumbo hip replacements are now contraindicated as a primary procedure due to metal wear and corrosion at the trunnion.
6. Huo MH, Stockton KG, Mont MA et al. What’s new in total hip arthroplasty? J Bone Joint Surg Am 2010;92-A:2959–2972.