This is an excellent question. Several people who come to us with end-stage knee arthritis say they have been 'rejected' by other orthopedic groups and told they needed to lose weight before surgery could be considered. Why is that? Most of the time the reason given is that the knee implant won't last because the excessive weight will cause premature loosening. Another reason is the complication rate is higher. Adverse events like infection and blood clots for example are supposedly more likely to occur. So what's the truth? Is this REALLY the case? Let's look at a couple of definitions first. In the medical world, weight is usually quantitated by a term called the 'BMI' or 'body mass index' which is calculated by the formula: weight x 708 divided by the square of the height in inches. Optimum weight for height is somewhere between 20 and 25 . Obesity is defined as anything over 30. Morbid obesity is defined as BMI over 40. Have there been any studies looking at weight and knee replacements? The answer is yes. Let's look at some of these:
1) 'The Outcome of Total Knee Arthroplasty in Obese Patients': 2004 article in The Journal of Bone and Joint Surgery (JBJS, p.1609)) by Foran et al out of Baltimore. They looked at 78 total knees done in 68 obese patients and compared with a matched group of non-obese over 6-7 years. They concluded that any degree of obesity (BMI >30) had a negative effect on the outcome in terms of greater complication rate and higher revision rate.
2) 'Total Knee Arthroplasty in Obese patients' (JBJS Stern and Insall 1990 Vol 72 p.1400): 2-5 year study looking at outcomes for 5 weight classes including obese and severely obese. Out of 257 knees, only 4 were fair or poor. No differences were found in the overall outcome scores over the 5 groups except there did appear to be a higher incidence of patellofemoral symptoms (knee-cap related) in the moderately and severely obese groups. "No clear-cut association was found between obesity and either thrombophlebitis or complications"
3) 'Does obesity influence the clinical outcome at 5 years following total knee replacement for osteoarthritis?':
a study by A.K. Amin et al from the British JBJS in Mar 2006. 370 knee replacements followed up to 5 years, comparing obese to non-obese patients. Conclusion was no statistically significant difference in complication rates. "Obesity did not influence the clinical outcome 5 years after knee replacement. "
Actually, several more studies are out there and overall the results are somewhat mixed as shown here, but in general it would appear that good to excellent results can be expected from knee replacement in spite of excess weight. Obesity is not an absolute contraindication for knee replacement although the risks of adverse events might be somewhat increased. Certainly it would be advantageous to lose weight for general health reasons, but my own experience is that behavior especially as we get older is extremely difficult to change and even more difficult in the setting of a painful arthritic joint. If an individual is overweight or obese and has an end-stage arthritic knee with intractable pain, most likely significant weight loss will not occur. The decision to proceed with knee replacement will likely depend on the surgeon's experience with this patient group, but as we have shown here, surgery can definitely be successful.