Medical Studies

Learning curve may be longer than thought for placing hip resurfacing components

 

It took surgeons 55 to 60 cases to get femoral components within ?5? of planned placement.

 

By Susan M. Rapp ORTHOPEDICS TODAY 2007; 27:12 May 2007

 

British and Australian researchers collaborating on a prospective study identified a longer-than-expected learning curve to accurately perform hip resurfacing arthroplasties.

 

Hip surgeons taking part in the study, all of whom had performed more than 1,000 hip surgeries, found they had to complete three-times more resurfacing surgeries than they expected in order to place the femoral hip resurfacing components within ?5? of the desired neck/head angle, said Diane L. Back, FRCS.

 

The surgeons had initially estimated their learning curve at 10 to 20 cases, Back told Orthopedics Today.

 

"The results actually showed that it took 55 to 60 cases for most of our surgeons to get the femoral component where they actually planned it," she said.

 

...Four surgeons participated Back and colleagues prospectively analyzed the first 100 hip resurfacing procedures of four consultant orthopedic surgeons, three from Australia and one from the United Kingdom.

 

They performed all procedures with the recently reintroduced Birmingham Hip Resurfacing System [Smith & Nephew]. The FDA approved the implant for sale in the United States last year.

 

All surgeons used standard instrumentation and a posterior approach. Investigators focused on femoral component position, but also analyzed notching and other complications.

 

To determine how accurately they placed the femoral components, surgeons first marked the ideal implantation site on preoperative radiographs. Investigators then calculated the corresponding neck/head angle.

 

...The longer-than-expected learning curve has huge implications for surgeons' success with this type of hip replacement, Back told Orthopedics Today.

 

"Hip resurfacing is good. I think that is well proven. You have to choose your right cases, but it does take you longer than you expect to learn. I think medico-legally it does have some implications."

 

For example, surgeons just learning the procedure should tell patients they are at the beginning of their learning curve and have not done many of the procedures, she said.

 

In the United States where orthopedists begin practicing after completing fewer hip replacements than surgeons in the United Kingdom or Australia, "It actually means their learning curve may take them 10 years to get out of," Back said.

 

Back said she wonders how, with the worldwide trend toward reduced orthopedic training hours, many new surgeons would be able to accrue enough cases to become proficient with the technique.

 

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Medium Term Serum Cobalt and Chromium Levels in Patients with Metal-metal Resurfacing

NOTE: This paper was presented at the New Trends of Joint Reconstructive Surgery 2002 Meeting, comparing serum levels in a control group, conventional THR, McKee Farrar, MOM Hip Resurfacing, and MOM THR (Sulzer), which concluded that metal ion levels are higher at 1 year with a small diameter MOM THR (Sulzer) than with resurfacing. As the Sulzer device has been in use for over 15 years with no associated problems reported to date, this is further evidence that this is currently not an issue.


V Reddy MD† J Dorairajan MD† J Nevelos PhD* S Krikler PhD FRCS(Orth)†.
† University Hospitals Coven try & Warwickshire NHS Trust * Corin Medical†

Introduction
The conservative principles of hip resurfacing combined with very low wear metal-metal bearing technology may provide a very attractive solution for the younger arthritic hip. The long-term effects of the inevitable metal ion release however, remain a concern. Serum ion levels from metal-metal resurfacing hip patients were measured pre-operatively and then every year for up to five years.

Materials and Methods
All patients had had either a McMinn (1995-1996) or Cormet® (1997-) hip resurfacing (both manufactured by Corin Medical, UK) implanted by a single surgeon. Blood was collected using a standardised technique to ensure no metallic contamination. Chromium analysis was by graphite furnace atomic absorption (ETA-AAS) using a 4100ZL or A800 Perkin Elmer instrument. Cobalt was analysed by inductively coupled plasma mass spectrometry (ICP-MS). Measurements were taken from 39 patients with 7 of those having bilateral resurfacings. Pre-operative levels were only taken from 15 patients, and these were universally low (below 20nmol/l).

Results – General Population
In all patients the serum levels of cobalt and chromium increased following resurfacing although the extent of the increase varied greatly between patients. There was a general decrease in ion levels from ~2 years. The data shows a definite trend of decreasing ion levels after 3 years. This may be consistent with running-in wear and healing of the peri-prosthetic tissues providing a smooth, stable joint. The decline in serum level seems to start slightly earlier in younger patients (<40) than older patients (>60).
 

Discussion – General Population

• Ion levels increase over the first two years before dropping to a generally low level.
• Younger patients’ ion levels peak earlier than older patients – probably related to activity level.
• There was no difference in ion levels between the McMinn and Cormet devices.
• Bilateral resurfacings did not appear to elevate the ion levels significantly.
• Insufficient sample numbers for statistical analysis.

Results – Outliers
The vast majority of patients studied had similar ion level release which remained below 250nl/l. There were, however three individual patients who had much higher ion levels which may cast light on possible causes of increased wear.

Case 1. Patient AP. 22 year old female at primary operation – Steep Acetabular Cup



Case 2. Patient LB. 42 year old female at primary operation Post operative Dislocation and Steep Cup



Case 3. Patient SM. 35 year old male at primary operation.






Discussion – Outliers
These cases were performed before the start of this study and so ion levels were not taken pre-operatively. Their levels were one or two orders of magnitude higher than the general population. (Note scale on the respective graphs). Also the ion levels show no sign of decreasing with time unlike the general population. Steep acetabular cup angles have been shown to be related to increased wear in hard-hard hip bearings (16) . Steep cups will also inevitably lead to an increased chance of dislocation (as in case 2). The large diameter heads used for hip resurfacing are naturally resistant to dislocation and Case 2 of this series remains the only dislocation to date in the senior author’s personal series of over 250 operations.

Conclusions
In the vast majority of patients with metal-metal hip resurfacing there is a small but early rise in serum metal ions over the first 2-3 years but the levels then gradually diminish over time .

Steep cups seem to be associated with increased levels, which could be due to wear and therefore increased ion release into the body.
 

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