Evaluation of Constant Thickness Cartilage Models vs. Patient Specific Cartilage Models for an Optimized Computer-Assisted Planning of Periacetabular Osteotomy

Abstract

Modern computerized planning tools for periacetabular osteotomy (PAO) use either morphology-based or biomechanics-based methods. The latter relies on estimation of peak contact pressures and contact areas using either patient specific or constant thickness cartilage models. We performed a finite element analysis investigating the optimal reorientation of the acetabulum in PAO surgery based on simulated joint contact pressures and contact areas using patient specific cartilage model. Furthermore we investigated the influences of using patient specific cartilage model or constant thickness cartilage model on the biomechanical simulation results. Ten specimens with hip dysplasia were used in this study. Image data were available from CT arthrography studies. Bone models were reconstructed. Mesh models for the patient specific cartilage were defined and subsequently loaded under previously reported boundary and loading conditions. Peak contact pressures and contact areas were estimated in the original position. Afterwards we used a validated preoperative planning software to change the acetabular inclination by an increment of 5° and measured the lateral center edge angle (LCE) at each reorientation position. The position with the largest contact area and the lowest peak contact pressure was defined as the optimal position. In order to investigate the influence of using patient specific cartilage model or constant thickness cartilage model on the biomechanical simulation results, the same procedure was repeated with the same bone models but with a cartilage mesh of constant thickness. Comparison of the peak contact pressures and the contact areas between these two different cartilage models showed that good correlation between these two cartilage models for peak contact pressures (r = 0.634 ∈ [0.6, 0.8], p < 0.001) and contact areas (r = 0.872 > 0.8, p < 0.001). For both cartilage models, the largest contact areas and the lowest peak pressures were found at the same position. Our study is the first study comparing peak contact pressures and contact areas between patient specific and constant thickness cartilage models during PAO planning. Good correlation for these two models was detected. Computer assisted planning with FE modeling using constant thickness cartilage models might be a promising PAO planning tool when a conventional CT is available.

Finite element analysis of mechanical behavior of human dysplastic hip joints: a systematic review

“…Patient-specific CT data for both normal and dysplastic hips were employed in several studies 42,43,45 . Others had only access to either normal 17 or dysplastic 15,47,48 patient-specific CT data and generated models of dysplastic or close-to-normal hips by deforming the acetabular rim or changing morphological parameters of the acetabulum through virtual osteotomy. One study employed patient-specific CT data of dysplastic hips pre-and post-osteotomy 16 .…”

“…CT-based models demonstrate bony anatomy well, but generating accurate geometries of soft tissues (cartilage and ligaments) is difficult since these structures are poorly seen on CT images. Some studies 42,43,45,48 used CT arthrography (CTA), which is invasive but clearly demonstrates articular cartilage (AC) boundaries, to directly model cartilage thickness (0.5e2.8 mm 45 , 0.8e2.0 mm 42,43 ). Non- x Includes only the acetabular cup.…”

“…However, Anderson et al 49 demonstrated that use of constant-thickness cartilage in normal hip FEM overestimated cartilage CP and underestimated contact areas. In dysplastic hips, Liu et al 48 found 21 ± 19% and 17 ± 14% error in simulated CP and area respectively when replacing patient-specific cartilage thickness with a constant-thickness cartilage layer. Therefore, utilizing patient-specific cartilage thickness likely enhances simulation accuracy 49 ; especially in dysplastic hips in which cartilage thickness is greater and more variable than in normal hips 50 .…”

“…The cartilage-labrum (Lb) boundary is usually not visible even in arthrograms. Studies either determine this based on morphology and reader judgment 17,42,43 , or simply disregard the Lb 15,16,45,47,48 . Several studies that included the Lb found its mechanical contribution in hip stabilization and load transfer from cartilage was greater (by 2e11 times) in dysplastic than normal hips.…”

Diagnosis and Management of Borderline Hip Dysplasia and Acetabular Retroversion

“…FEA has been used very similarly to DEA to evaluate joint contact stresses in normal versus dysplastic hips [81][82][83][84] and to evaluate or guide surgical interventions based on reduction in contact stress. [85][86][87][88][89][90] Modeling overcorrection and residual impingement after PAO can best be performed using FEA, 86 which allows for incorporation of the mechanically complex soft tissues 91 that are at highest risk of developing high stresses and strains and ultimately progressing to failure. 81 A recent FEA modeling study of a small number of acetabular retroversion patients found that while PAO served to offload forces passing through the labrum during several activities of daily living and shift contact stress medially and posteriorly within the articular surface of the acetabulum, peak contact stress in these patients actually increased postoperatively.…”