Morbid obesity may increase dislocation in total hip patients: a biomechanical analysis

Abstract

Background: Obesity has reached epidemic proportions in the United States. Recently, obesity, especially morbid obesity, has been linked to increased rates of dislocation after THA. The reasons are unclear. Soft tissue engagement caused by increased thigh girth has been proposed as a possible mechanism for decreased joint stability.

Questions/purposes: We asked (1) whether thigh soft tissue impingement could decrease THA stability, and if so, at what level of BMI this effect might become evident; and (2) how THA construct factors (eg, head size, neck offset, cup abduction) might affect stability in the morbidly obese.

Methods: The obesity effect was explored by augmenting a physically validated finite element model of a total hip construct previously comprising just implant hardware and periarticular (capsular) soft tissue. The model augmentation involved using anatomic and anthropometric data to include graded levels of increased thigh girth. Parametric computations were run to assess joint stability for two head sizes (28 and 36 mm), for normal versus high neck offset, and for multiple cup abduction angles.

Results: Thigh soft tissue impingement lowered the resistance to dislocation for BMIs of 40 or greater. Dislocation risk increased monotonically above this threshold as a function of cup abduction angle, independent of hardware impingement events. Increased head diameter did not substantially improve joint stability. High-offset necks decreased the dislocation risk.

Conclusions: Excessive obesity creates conditions that compromise stability of THAs. Given such conditions, our model suggests reduced cup abduction, high neck offset, and full-cup coverage would reduce the risks of dislocation events.

Fig. 1

During hip adduction, a laterally directed force arises owing to thigh-to-thigh contact, acting to push the femoral head from the cup, compromising joint stability. (Modified and reprinted with permission of Lippincott Williams & Wilkins from Kim Y, Morshed S, Joseph T, Bozic K, Ries MD. Clinical impact of obesity on stability following revision total hip arthroplasty. Clin Orthop Relat Res. 2006;453:142–146.)

Fig. 2

The FE model of extraarticular soft tissue contact and subsequent THA instability consisted of (1) implanted THA hardware, (2) hip capsule (posterior ½ of the capsule is here rendered transparent for clarity), and (3) extraarticular soft tissue including muscle, adipose tissue, and skin, surrounding a rigid femoral canal. The model assumed right-left symmetry of the native anatomy. E = Young’s modulus; K = bulk modulus; ε = nominal strain; ν = Poisson’s ratio.

Fig. 3

Eight graded levels of BMI were simulated. Anatomic shape parameters were used for the baseline case (BMI = 20 kg/m²), which then were scaled using anthropometric data [31] to model thigh geometry for an overweight patient (BMI = 25 kg/m²) and six grades of obesity. An initial analysis step brought the thighs into nominal apposition (as shown) for the beginning of the analysis, resulting in contact occurring between the thighs in all six obese FE models.

Fig. 4A–B

The graphs show the hip-center-to-hip-center distance as measured from standard AP radiographs in (A) 113 healthy women and (B) 33 healthy men. Subjects with degeneration or anomalies in the hip and pelvic anatomy were excluded. The center of the femoral head was defined as the center of the circle best fitting the bony contour of the femoral head. Interhip center distance was defined as the distance between the centers of femoral heads, corrected for radiograph magnification. Considering the radiographic data shown here, along with cadaveric measurement, a median interhip separation distance of 200 mm was inferred and used in all FE simulations.

Fig. 5A–D

(A) Physical corroboration of thigh-to-thigh contact computation was performed using an interface pressure mat applied between the thighs of an obese subject (without THA) performing a sit-to-stand maneuver. (B) Spatial integration of the Tekscan mat contact stress measured during thigh impingement resulted in 146.4 N of peak lateral load. Integration of contact stresses from the corresponding portion of the FE-computed thigh-to-thigh contact region (anatomic constraints precluded positioning the mat to cover the entirety of the contact region) provided a basis for direct comparison. Physical representation of the placement of the pressure mat is shown for the (C) 35- and (D) 40-kg/m² BMI FE models. (The peak thigh circumference of the study subject fell between these two simulated cases.) Peak contact force computed in the approximate area covered by the sensing mat (dashed boxes) was 170 N (approximately a 16% discrepancy versus experimental) for the 35-kg/m² BMI case and 250 N for the 40-kg/m² BMI case.

Fig. 6

Instability (quantified in terms of femoral head subluxation distance) in the simulated obese THA models occurred during hip adduction, at the terminal stage of the sit-to-stand maneuver. Contact between the thighs, the intensity of which increased during hip adduction, caused development of a laterally directed force whose magnitude was sufficient to cause joint instability, leading to frank dislocation in the highest BMI models, as illustrated for a 50-kg/m² BMI simulation.

Fig. 7A–C

(A) Dislocation risk was sensitive to increased BMI and increased cup abduction for the 28-mm head, although substantial instability (subluxation > 1 mm) did not occur for simulated BMIs of 35 kg/m² or less. Increased cup abduction also led to elevated dislocation risk. (B) A generally similar BMI-versus-cup-abduction relationship was seen for the standard-offset 36-mm cup, although instability was somewhat more pronounced at higher cup abduction angles. (C) However, dislocation risk was reduced substantially when a high-offset (8 mm) neck was used with the 36-mm cup. Subluxation for the high-offset neck occurred only for the most extreme values of BMI and cup abduction and even then remained far below the level required for frank dislocation. Deg = degrees.

Fig. 8

A graph shows the thigh-to-thigh contact forces required to cause dislocation for a 28-mm cup positioned in 60° abduction, versus cup edge radius. Required thigh-to-thigh contact forces were sensitive to cup edge geometry. Cups with larger edge radii (and therefore less articular head coverage) required smaller laterally directed thigh-to-thigh contact force to become unstable.

Fig. 9A–B

(A) The 28-mm THA hardware consisted of a standard-offset neck with a 5° anteverted stem (left), a 28-mm-diameter cup (middle) with a flat lip and chamfer (right), and 180° head articular coverage, resulting in 14 mm of jump distance being required for dislocation. (B) The 36-mm THA hardware also consisted of a standard-offset neck anteverted to 5° (left). The cup diameter was 36 mm (middle), but the rounded lip and chamfer of the cup (right) resulted in only 163° articular coverage, decreasing the required jump distance from a full head diameter (18 mm) to only 15.3 mm.