Is a short stem an option for obese patients?

Introduction

Obesity contributes to a higher rate of osteoarthritis in joints not only because of cartilage loading forces but also because the adipose tissue releases adipokine, a protein that leads to excess cartilage inflammation and degradation [11] Stihsen C. et al. Subsidence of a cementless femoral component influenced by body weight and body mass index. International Orthopaedics (SICOT) 2012; 36:941-947.. Thus the risk to achieve osteoarthritis and to be in need of a total hip arthroplasty (THA) is up to 8.5 times higher than in non-obese patients [1] AAHKS: OrthoCarolin Hip and Knee Center: A workgroup of the American Association of Hip and Knee Surgeons (AAHKS) Evidence Based Committee. Obesity and total joint arthroplasty, a literature based review. The Journal of Arthroplasty 2013: 28:714-721;.

The use of short stems in THA has increased over the last 10 years. There is a big variety of short stems that have different philosophies. The benefits described are a more physiological load transfer to the proximal femur and preservation of bone. They enable soft tissue as well as muscle protection and facilitate minimally invasive implantation technique. The smaller implant-bone contact surface may cause inferior primary stability and be associated with higher migration rates compared to traditional stems. Biomechanical studies of the University in Ulm though could show that the stability is comparable to standard stems [2] Bieger R. et al. Biomechanics of a short stem: in vitro primary stability and stress shielding of a conservative cementless hip stem. Journal of orthopedic research month 2013, published in Wiley Online Library.. The question is if a short stem is suitable also for obese patients.

Our experience

Weight limits for implants are not often expressed by the manufacturer. Especially with short stems surgeons are skeptic to use them with heavy and obese patients. We took part in a multi-center study in which the stem “optimys” (Figure 1) was used that follows the calcar and slips into the femur easily. 483 patients were divided into 3 groups with body mass index (BMI) <25, 25-30, >30. After 24 months there were no statistically significant differences between these groups respective the implant. As also can be seen in standard implants there were very few subsidences in all three groups up to 6mm during the first three months mainly because the chosen implant was too small partly due to the learning curve with this new implant. Later on during the follow up the implants were stable and no more subsidence was evident (Figure 2).

Major systemic and wound healing problems were seen more often in the obese patients (Figure 3). But no significant differences were seen between the classes respective the clinical outcome. Pain at rest and under load was measured with the visual analog scale (VAS) (Figure 4).

The measured difference in the Harris Hip Score is very little and due to the general restrictions by obesity (Figure 5). In another evaluation of this study group we divided 773 patients into weight classes of <100 and >100 kg. Again there were no significant differences to be seen. Pain at rest and under load was measured with the visual analogue scale. The heavy patients complained slightly more and were less satisfied achieving 94.4 points in the Harris Hip Score vs 97.7 points in the lighter group (Figure 6).

Discussion

Literature is rare on this topic. A clinical study about the influence on migration patterns of short stems in THA [5] Floerkemeier T. et al. The Influence of tribological pairings and other factors on migration patterns of short stems in THA, Hindawi: BioMed Research Interantional Volume 2017, Article ID 8756432. revealed that weight or the size of the femoral stem did not affect the migration of the stems. A study by Freitag et al. [6] Freitag T. et al. Migration pattern of a femoral short-stem prosthesis: a 2-year EBRA-FCA-study, Arch Orthop Trauma Surg 2014 Jul; 134(7): 1003-8. of 72 hips in patients with a mean BMI of 29 (21-51) used EBRA technique to measure the subsidence of a short stem. They report subsidence of 1mm (+1.4mm) after 24 months and conclude, that BMI, gender and implant offset did not influence implant migration on a statistical significant level. Nevertheless, a tendency towards more migration in obese and female patients was observed. Stihsen et al. [12] Traina F. et al. Patient weight more than body mass index influences THA long term survival, Hip Int 2011: 21(06):694-699. studied subsidence of the stem Vision 2000 respective weight or BMI of the patient. The study demonstrates that BMI does not trigger progressive stem migration whereas weight above 75 kg and height over 165 cm may trigger increased stem migration. The idea is that each kg puts stress onto the implant and may increase the risk of migration. Thus a tall man with normal BMI puts much more stress onto the implant than a small woman with a very high BMI. Advice is given to the surgeon to aim to fit the prosthesis as tight as possible. That weight more than BMI influences survivorship is also the conclusion of Traina et al. [12] Traina F. et al. Patient weight more than body mass index influences THA long term survival, Hip Int 2011: 21(06):694-699. especially for men above 80kg. More attention should be given to this aspect in studies and by manufacturers who should test and determine weight limits for their implants. McLaughlin [10] Pottie P. et al. Obesity and osteoarthritis: more complex than predicted! Ann Rheum Dis 2006;65(11): 1403. collected data of 285 uncemented Taperloc femoral components after 10-18 years and compared the outcome in obese and non-obese patients. He identified no statistically significant difference with regards to clinical and radiological outcome or complications.

There are quite a few studies that deal with obesity in general and joint arthroplasty. They describe higher complication rates of local inflammations or infections. Gastrointestinal and respiratory tract problems are reported to be higher in morbidly obese patients [7] Friedmann R.J. et al. Complication rates after hip or knee arthroplasty in morbidly obese patients, Clin Orthop Relat Res 2013; 471:3358-3366.. A review of literature [1] AAHKS: OrthoCarolin Hip and Knee Center: A workgroup of the American Association of Hip and Knee Surgeons (AAHKS) Evidence Based Committee. Obesity and total joint arthroplasty, a literature based review. The Journal of Arthroplasty 2013: 28:714-721; also points out the increase of risk for local and systemic complications especially because of the comorbidities such as diabetes, obstructive sleep apnea, thromboembolic disease... However there is no significant effect of obesity on the surgery satisfaction and higher BMI is not associated with worse outcomes of the hip arthroplasty. Milhalko et al. state that “no upper BMI limit has been established beyond which orthopedic surgery is contraindicated, and each patient should be considered individually. Overweight or obese patients should be counseled about the added risks of surgery and offered resources for weight loss before surgery” [9] Milhlako W.M. Obesity, orthopaedics and outcomes, J. Am Acad Orthop Surg 2014; 22:683-690.

Conclusion

Our findings and those in literature suggest there is no evidence to withhold THA from obese patients with arthritic hips. The impact of complications and comorbidity is considerable. Of course it is a demanding operation and should be performed by high volume surgeons. Surgery is more difficult and of longer duration with the risk of malpositioning the components [4] Elson L.C. et al. Are morbidly obese patients undergoing THA at an increased risk for component malpositioning? The Journal of Arthroplasty 2013: 28:41-44. especially with minimally invasive techniques. The selected implant needs to have the ideal size and optimal pressfit. In order to prevent dislocation, enough offset and full-cup coverage is needed [3] Elkins J.M. et al. Morbid obesity may increase dislocation in total hip patients: A biomechanical analysis, Clin Orthop Relat Res 2013; 471:971-980. and on the acetabular side double mobility cups are more effective than big heads to reduce the risk of dislocation [8] Maisongrosse P. et al. Obesity is no longer a risk factor for dislocation after THA with a double-mobility cup. Int Orthop 2015 Jul;39(7):125-128.. In conclusion short stems seem to perform as well as standard stems. Further long term studies have to prove this statement.