Biomechanical and clinical assessment of dissociation in bipolar hip hemiarthroplasty

Dissociation is a special type of dislocation that is rarely seen in bipolar hemiarthroplasty.

To investigate the clinical and biomechanical causes of dissociative dislocation of acetabular cup components in the hips of patients after bipolar hemiarthroplasty (BHA).

BHA heads were divided into three groups according to their design. Cam-out and pull-out biomechanical tests were conducted to investigate the separation strengths of the BHA heads.

Among the 1684 BHA surgeries performed, the revision surgery rate was 4% (68 hips) and dissociation rate was 0.8% (15 hips). According to the cam-out test result, the highest values in the parameters ultimate force (F_max) ultimate torque (T), vertical displacement at maximum force (δ_m), rotation at maximum force (ϕ_m), maximum principal strain (Ɛ_max), minimum principal strain (Ɛ_min), average normal strain (Ɛ_av), maximum shear strain (γ_max), uniaxial normal strain (Ɛ_un) were detected in Type 2 BHA head. The pull-out values of the implants were sufficient to prevent the distraction force that may occur on the implant due to manual traction applied to the hip dislocations. However, in closed reduction maneuvers, cam-out-shaped deformation forces may cause dissociation in Types 1 and 3, but not in Type 2 BHA heads. According to the pull-out test results, while high values were detected in the parameters Fmax and stiffness (k) in the Type 2 BHA head, δm and maximum force (E) parameters were found to be high in the Type 1 BHA head. In the cam-out test, a strong positive relationship was found between the thickness and width of the polyethylene locking ring and F_max, T, δ_m, ϕ_m, Ɛ_max, γ_max, Ɛ_un.

Better BHA head designs and polyethylene designs may help resolve the rare problem of dissociation, which almost always leads to resurgical procedures.