Researchers find thigh size behind hip implant failure
Thigh size in obese people may contribute to the failure of their hip implants, a new international study shows. Led by the University of Iowa in the United States, researchers simulated hip dislocations and observed that the bigger the girth in a morbidly obese person's thigh, the more likely their hip will become instable.
Presented in the journal Clinical Orthopaedics and Related Research, the study suggests that surgeons should modify surgical procedures to mitigate the risk of hip dislocation in obese patients. The researchers also note that other designs for hip replacement implants should be considered.
'We have shown that morbidly obese patients' thighs are so large that they are actually pushing each other outward and forcing the implant out of its socket,' said lead author Jacob Elkins, a graduate student at the University of Iowa. 'Studies have shown up to a 6.9-fold higher dislocation rate for morbidly obese patients compared to normal weight patients.'
People who suffer from debilitating hip joint pain benefit from total hip replacement because it gives them back their mobility. Data from the European Commission Joint Research Centre, Institute for Health and Consumer Protection show that around one million people undergo total hip replacement surgery each year, and from the National Institute of Arthritis and Musculoskeletal and Skin Disease (NIAMS) in the United States that more than 230,000 total hip replacements are performed in the United States each year. Over 90 % of the latter do not require follow-up repair or replacement.
Problems surface, however, when a hip replacement fails. It is painful for the patients, both physically and financially. Dislocation is the number one cause for failed implants, according to Medicare hospital discharge data. A hip implant is a ball-in-socket mechanism. It works just like a human hip joint but it does not have the connective tissue that stabilises a normal hip joint. Because of this, the replacement's ball portion can 'pop out' at times.
Mr Elkins developed a computational model that the team used to determine how a hip implant works in patients. They evaluated the effects of thigh-on-thigh pressure on the hip implant during a wide range of movements from sitting to standing. The researchers also studied how implants performed in various body types, using a hip-centre-to-hip-centre distance of 200 millimetres as a basis for their analyses of thigh girth for 8 different body mass indexes (BMI, between 20 and 55, with 40 being for morbidly obese). They observed that thigh soft tissue impingement raised the risk of dislocation for BMIs of 40 or greater; implants with a larger femoral head diameter did not substantially improve joint stability; and an implant with a high-offset femoral stem decreased the dislocation risk.
'The larger your legs are, the more force that goes through the hip joint,' Mr Elkins explained. 'It's a simple concept. When your thighs are real big, they push on the hips.'
So with respect to surgical treatments, the researchers believe surgeons should use the study's results to determine the optimal implant design for each respective patient.
'The number one thing surgeons can do is what is called a 'high offset femoral stem,'" says senior author Professor Thomas Brown of the University of Iowa. 'Basically, the implant's femoral stem is longer, so it effectively shifts the leg further away from the centre rotation of the joint. The thighs then would need to move even further inward before they would abut one another and generate the forces necessary for dislocation.'
Experts from the Czech Technical University in Prague, Czech Republic contributed to this study.
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Data Source Provider: Clinical Orthopaedics and Related Research; University of Iowa
Document Reference: Elkins, J.M. et al. 'Morbid obesity may increase dislocation in total hip patients: a biomechanical analysis', Clinical Orthopaedics and Related Research, 2012. doi:10.1007/s11999-012-2512-3
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