Aims

Type 2 diabetes mellitus (T2DM) impairs bone strength and is a significant risk factor for hip fracture, yet currently there is no reliable tool to assess this risk. Most risk stratification methods rely on bone mineral density, which is not impaired by diabetes, rendering current tests ineffective. CT-based finite element analysis (CTFEA) calculates the mechanical response of bone to load and uses the yield strain, which is reduced in T2DM patients, to measure bone strength. The purpose of this feasibility study was to examine whether CTFEA could be used to assess the hip fracture risk for T2DM patients.

Background. Periprosthetic femoral fractures following total hip arthroplasty are relatively uncommon but are associated with significant morbidity. With an increasing number of total hip arthroplasties being carried out in an aging population we need to ensure correct implants are chosen for our patients. A recent review of NJR data suggested a significantly higher revision risk for the Zimmer CPT stems due to periprosthetic fractures when compared to the Stryker Exeter stems. Objectives. Our aim was to compare the biomechanics of periprosthetic fractures around the CPT and Exeter V40 stems in a composite saw bone model to identify if a difference in fracture risk exists between the two stems. We also compared the engineering design of the two implants in order to analyse the possible effect this may have on fracture risk. Study Design & Methods. Fourteen composite femurs were divided into two groups and cemented using Palacos R cement with either the CPT or Exeter V40 stem by a single surgeon. The implanted femurs were then mounted onto an Instron machine and were axially loaded and torqued to fracture with an axial compressive force of 2000N over 10 seconds followed by a rotation of 40 degrees applied over 1 second. A power calculation from a previous composite saw bone model study suggested that a minimum of 6 implanted femurs would be required in each group. Results. The implanted femurs invariably sustained fracture patterns similar to the Vancouver B2 periprosthetic fracture which are commonly seen in clinical practice. Implanted femurs with CPT stems suffered periprosthetic fractures with less rotation when compared to those femurs with the Exeter V40 stem (20.10 versus 33.60, p<0.01). We also found that CPT implanted femurs were fracturing at significantly lower torque values when compared to the Exeter V40 implanted femurs (124Nm Versus 174Nm, p<0.01). The energy release rate (G111) for CPT stems was 21.8Nm compared to 61.2Nm for Exeter V40 stems. The higher energy release with Exeter stems led to more comminuted fractures in Exeter implanted femurs when compared to the CPT femurs, which fractured earlier, but with simpler fracture patterns. Finite element method (FEM) simulation analysis showed that fractures initiated between the prosthesis and cement at the proximal end of the femur. Two dimensional sections at the same height showed a difference in bone-cement-implant geometrics at the critical point of failure suggesting that a design cause may be the reason for the higher risk of periprosthetic fractures in CPT implanted femurs. Conclusions. Our observations may explain the higher revision risk secondary to periprosthetic fractures that has been observed with the CPT stem when compared to the Exeter V40 stem

In this study we used subject-specific finite element analysis to investigate the mechanical effects of rotational acetabular osteotomy (RAO) on the hip joint and analysed the correlation between various radiological measurements and mechanical stress in the hip joint.

We evaluated 13 hips in 12 patients (two men and ten women, mean age at surgery 32.0 years; 19 to 46) with developmental dysplasia of the hip (DDH) who were treated by RAO.

Subject-specific finite element models were constructed from CT data. The centre–edge (CE) angle, acetabular head index (AHI), acetabular angle and acetabular roof angle (ARA) were measured on anteroposterior pelvic radiographs taken before and after RAO. The relationship between equivalent stress in the hip joint and radiological measurements was analysed.

The equivalent stress in the acetabulum decreased from 4.1 MPa (2.7 to 6.5) pre-operatively to 2.8 MPa (1.8 to 3.6) post-operatively (p < 0.01). There was a moderate correlation between equivalent stress in the acetabulum and the radiological measurements: CE angle (R = –0.645, p < 0.01); AHI (R = –0.603, p < 0.01); acetabular angle (R = 0.484, p = 0.02); and ARA (R = 0.572, p < 0.01).

The equivalent stress in the acetabulum of patients with DDH decreased after RAO. Correction of the CE angle, AHI and ARA was considered to be important in reducing the mechanical stress in the hip joint.

Cite this article: Bone Joint J 2015;97-B:492–7.