| Abstract|| |
Background: The management of unsalvageable femoral head conditions in the young patient has remained an unresolved dilemma. Articular surface replacement of the hip has recently made some headway in terms of providing near-normal hip joint mechanics and function. However, this surgery has been limited to early stages of arthritis only with reasonable maintenance of head-neck congruity and morphology. Femoral neck fractures, osteonecrosis with large segment collapse, advanced arthritis with femoral incongruity, etc are traditional contraindications to the resurfacing technique.
Methods: We present here a report on our series of 20 cases of unsalvageable femoral heads in young patients (age range, 27 to 52yrs), over a twelve month period (Aug 2004 to Jul 2005), treated with the unipolar ASR prosthesis. Fifteen patients (two had bilateral hip pathology) had primary or secondary arthritis (degenerative, post-traumatic, ankylosing spondylitis and postavascular necrosis) while three had old operated femoral neck fractures. All patients underwent hip replacement surgery using the Unipolar ASR prosthesis.
Results: Clinical and radiological results at 6-month follow up have been very encouraging and warrant further study. At an average of 4 months post-operatively, patients were able to squat, sit on the ground and perform light sporting activities.
Conclusions: The Unipolar ASR prosthesis is an extension of the articular resurfacing technique employing similar principles (large size bearings, metal-on-metal interfaces), and has incorporated the advantages of the uncemented technique. We propose that this technique be more frequently used so as to brighten the prognosis of the young active patient with unsalvageable hip conditions, especially in the Asian scenario.
Keywords: Unsalvageable femoral head; Unipolar ASR prosthesis; Surface replacement.
|How to cite this article:|
Marya S, Thukral R. The unipolar ASR : Viable option in unsalvageable femoral head conditions in the young patient. Indian J Orthop 2006;40:70-3
|How to cite this URL:|
Marya S, Thukral R. The unipolar ASR : Viable option in unsalvageable femoral head conditions in the young patient. Indian J Orthop [serial online] 2006 [cited 2019 Jan 16];40:70-3. Available from: http://www.ijoonline.com/text.asp?2006/40/2/70/34442
| Introduction|| |
Surface hip replacement, a bone-conserving alternative to total hip replacement, is a significant development in the evolution of hip arthroplasty  . With a surface replacement, only the surface of the joint and a few millimeters below this are removed. These surfaces are replaced with a thin layer of metal, leaving the rest of the bone intact. The advantages are that the femoral head and neck are retained and thus no femoral stem prosthesis is necessary, and as a consequence, revision to a total hip when required is technically easy  . As younger patients need surgery for hip arthritis, articular resurfacing surgery seems the ideal treatment, and is suggested in young people with degenerative hip disease, congenital hip dysplasia or Perthes' disease, with avascular necrosis and ankylosing spondylitis as extended indications  . The greatest limitation is that it cannot be performed in conditions with head-cup size mismatch, or large head defects, or unsalvageable head conditions (collapse, fracture neck, etc.)  .
Medium-term results from the McKee-Farrar metal on metal articulation, and short-term and medium-term results from contemporary metal-on-metal hip replacements have been very encouraging ,, , and form the basis for the new metal-on-metal, large bearing surface replacement prostheses. Articular surface replacement of the hip has made some headway in terms of providing near-normal hip joint mechanics and function. Modern metal-on-metal hip resurfacing was introduced as a less invasive method of joint reconstruction for the younger and more active group. Following the reintroduction of metal-on-metal articulating surfaces for total hip arthroplasty in Europe in 1988, Amstutz et al developed a surface arthroplasty prosthetic system using a metal-on-metal articulation 6 . The first 400 hips treated with metal-on-metal hybrid surface arthroplasties at an average follow-up of three and a half years showed that the majority of the patients returned to a high level of activity, including sports, and 54% had activity scores greater than 7 on the University of California at Los Angeles (UCLA) activity assessment system  . Kaplan-Meier survivorship curves demonstrated that the rate of survival of the components at four years was 94.4%. No femoral radiolucencies were observed among the hips in which the metaphyseal stem was cemented. The early designs of these surface replacement hips now have 12-year survival figures that are comparable to cemented hip designs in the young  .
However, femoral neck fractures, and other unsalvageable femoral head conditions (viz. osteonecrosis with large segment collapse, advanced arthritis with femoral incongruity, etc) have limited this surgery to early stages of arthritis with relatively preserved femoral head congruity and morphology  . A modification of the articular resurfacing technique (with the primary objectives of providing longlasting hips, permitting near-normal movements and using uncemented method of fixation) promises to extend these advantages to younger patients with unsalvageable femoral head conditions.
| Materials and methods|| |
Twenty cases of unsalvageable femoral heads in 18 young patients were treated with the unipolar ASR prosthesis, over a twelve month period (Aug 2004 to Jul 2005). There were 12 males and 6 females. The diagnosis was avascular necrosis (with > 1cm. collapse) in 9 patients (one had bilateral unipolar ASR replacement consequent to severe bilateral femoral head destruction). Two patients had severe rheumatoid arthritis (RA). One patient underwent bilateral unipolar ASR prosthesis for near-total bilateral hip ankylosis (ankylosing spondylitis). Six patients had secondary osteoarthritis with femoral head collapse (old healed acetabular fractures in 4, and femoral head collapse following fixed femoral neck fractures in 2 patients). The average age of the patients was 39 (age range, 27 to 52 years). In all cases, the femoral head was severely damaged or absent (unsalvageable, hence excluding the possibility of articular surface replacement) [Figure - 1]a. The criteria used to define the unsalvageable femoral head were femoral head defects > 1cm, limb shortening > 1cm, head-cup size mismatch (seen on preop radiological templating), wandering acetabulum (all incidentally contra-indications for the resurfacing technique). Desire of the patient for excessive hip movements was a prerequisite, as are cost constraints.
All patients were fitted with an uncemented 'Corail' femoral stem, attached to the unipolar ASR femoral head prosthesis with a taper sleeve adaptor [Figure - 1]b. We used a curved posterolateral skin incision and the anterolateral modified Hardinge approach. Post-operatively, the patients were allowed nil weight bearing (NWB) crutch walking at day 3, partial weight bearing (PWB) at 6 weeks, and full weight bearing (FWB) walking with stick at 3 months ((for the unilateral cases). The 2 patients with bilateral unipolar ASR prosthesis were allowed in-bed mobilization for 6 weeks, then started on PWB walking with crutches, and permitted FWB walking without support after 3 months. Between 3 and 4 months post-operatively, patients were permitted to sit crosslegged, squat and indulge in driving and light sports (golf, table tennis).
| Results|| |
We had no cases of infection, DVT, hemorrhage or dislocation. There were two instances of femoral shaft splitting during insertion of the uncemented 'Corail' stem, but these were managed with cerclage wiring [Figure - 2]a,b. This did not have any bearing on the end result in these patients.
Patients were evaluated at end of 3 months, 6 months and 12 months post-operatively. Average follow-up was 9 months (range, 6 months to 18 months). We evaluated patients based on a modified scoring system [Table - 1] taking into account Harris Hip scores, the return to normal hip functions and advanced hip functions (car driving and light sports). We achieved excellent early results in 16, and good results in 2, with all patients being able to sit cross-legged and squat. A detailed long-term follow-up of these patients is being carries out and will be reported in time.
| Discussion|| |
Total hip arthroplasty is less than ideal in young patients with hip arthritis. Surface hip arthroplasty offers a somewhat conservative treatment for this problem. The theoretical advantages are minimal bone resection, normal femoral loading, avoidance of stress shielding, maximum proprioceptive feedback, restoration of normal anatomy (offset, leg length & version), minimal risk of dislocation and easy revision when required ,, . Operative surgery for surface replacement is straightforward, requiring little preoperative planning and immediate weight-bearing postoperatively. Should failure occur, little bone stock loss is incurred and revision to a total hip replacement is as simple as primary hip arthroplasty ,, . There is no significant difference in the length of time needed to perform the surgery and no difference in the length of hospital stay if compared to a similar young group of patients having a more conventional hip replacement. Resurfaced femurs have shown 12-year survival rates close to 97% in the recent reports published, and long-term results are awaited  .
The metal-on-metal articulation has been established as a promising alternative in young patients in both older (McKee-Faraar hips) and contemporary studies. In a retrieval study on cobalt-chrome alloy McKee-Farrar (metal-on-metal) matching acetabular and femoral components at revision total hip arthroplasty at an average time of 16 years, the findings showed very low wear and loss of sphericity  . Polishing wear (type 1), fine abrasive (type 2), multidirectional dull abrasive (type 3), and unidirectional dull abrasive wear (type 4) of the articulating surfaces were identified. The mean percent area of femoral heads occupied by types 2, 3, and 4 wear was 18%, 5%, and 2%, respectively  . More recent analyses of the early series have shown the advantages of metal-on-metal to be better and have led to a renaissance of this articulation. Initially, stainless steel was used because it was easy to manufacture and polish. Current metal-on-metal bearings are based on cobalt-chromium-molybdenum alloys with varying carbon contents. Such bearings are self-polishing. Linear wear rates remain at the level of a few micron a year. An improvement in technology has increased the life span of the metal-on-metal THR-bearing system. This bearing concept probably permits the use of larger head sizes, to reduce the risk of impingement and luxations  . The low frictional torque from the metal on metal bearings is entirely consistent with the clinical experience of historic metal on metal joints having lasted 30 years or more  . With metal-on-metal bearings, the volumetric wear has been reduced 20-100 times from those with polyethylene, and there is no penalty for the large ball size  . These devices are now conservative on the acetabular as well as femoral side. Hybrid or all-cementless fixation is arguably superior to earlier all-cemented devices. In a series on cementless articular resurfacings by Amstutz et al  , the results up to 4 years have been complication-free, with an absence of pain and a return to high functional levels, including participation in sports. Although follow-up was short, surface replacement with the large ball size was found extremely stable, and dislocation rare .
Reported complication rates have also been low. There has been no report of dislocation of the resurfaced hip in literature. Dislodgement of the acetabular component has been observed, possibly due to inadequate primary fixation, and which is easily correctable by appropriate component repositioning. Markedly reduced amounts of fat and marrow are seen on trans-esophageal echocardiography after resurfacing compared to conventional stem total hip replacement, thereby limiting thromboembolic complications. A recently published study  demonstrates the risk of osteonecrotic femoral head collapse following articular resurfacing surgery. In a study of 377 patients who underwent resurfacing arthroplasty, 13 were found to require revision; for fracture of the femoral neck in eight, loosening of a component in three and for other reasons in two. None of these cases had shown histological evidence of osteonecrosis in the femoral bone at the time of the initial implantation. At revision, bone from the remnant femoral head showed changes of patchy osteonecrosis in nearly all cases  .From these established reports, important risk factors that have been identified for femoral component loosening are large femoral head cysts, patient height, female gender, and smaller component size in male patients  . Arthritic hips of limbs that are more than 1 cm shorter than the contralateral limb or those that have a comparatively low horizontal femoral offset are biomechanical limitations in the selection of hips for resurfacing  .
The principles of the 'in-vogue' resurfacing arthroplasty are metal-on-metal articulation, large bearing surfaces and initial sturdy fixation. It only seems logical that an extension of these principles in unsalvageable femoral head conditions would lead to similar exciting results. The unipolar ASR femoral prosthesis is based on this belief. This prosthesis is an extension of the articular surface replacement technique employing similar principles (large anatomical size bearings, metal-on-metal interfaces) with proper use of the uncemented technique of fixation. Hips where the femoral head contains large collapsed areas / defects, femoral head fractures, coax vara more than 90 degrees and weakened cystic femoral head/ necks , (all primary contra-indications for resurfacing) are amenable to this prosthesis. Anticipated intra-operative limitations, such as inability to revise to smaller components, the risk of neck notching leading to fractures, and great limitation in cup-head size matching, are no longer applicable with the unipolar ASRO femoral prosthesis. However, factors such as patient selection, surgical technique, and durable fixation of the components remain critical as reliance is on uncemented fixation. The long-term survival reports of the McKee-Faraar metal-on-metal hip replacement testify to the promising future of this anatomical Unipolar ASR prosthesis. However, long-term studies of this prosthesis are not available at present.
There is another major concern about the extent and duration of the relevant "internal" exposure to Cr and Co ions. It is yet unclear whether the medium-term corrosion rate is high or, on the contrary, it becomes negligible, because of the continuous surface finishing. This exposure should be carefully monitored, in order to clarify the biologic effects of ion dissemination and, consequently, to identify risks concerning long-term toxicity of metals  .
With time, resurfacing hip replacement is providing fantastic opportunities to the young arthritic to lead a nearnormal life. However, the experience and results with the unipolar ASR prosthesis is very limited. Preliminary experience identifies definite early advantages, and simultaneously underscores the importance of obtaining secure initial fixation. This metal on metal hip surface replacement procedure seems to be the answer to the burning issue of hip replacement in the young active population, where the femoral head is non-salvageable. We however need longterm evaluation and are waiting eagerly for medium and long term results to justify our excitement.
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Director Orthopedics & Institute of Joint Replacement, Max Hospital, 2, Press Enclave Road, New Delhi – 110017
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2]
[Table - 1]