Finite Element Analysis of Proximal Cement Fixation in Total Hip Arthroplasty

Total hip arthroplasty (THA), or surgical replacement of the hip joint with a prosthesis, is a reconstructive procedure that has improved the management of hip joint diseases that have responded poorly to conventional medical therapy. There are reasons to believe that the proximal part of the cement withstands more stress than the distal part in THA. Therefore, this study aims to determine whether it is possible to perform THA by cementing only the proximal part of the hip prosthesis. The polymethylmethacrylate cement has a Young’s modulus of 2GPa, a tensile strength of 29 MPa, and a Poisson’s ratio of 0.3. This analysis was done using a stainless steel stem model provided by the Department of Orthopaedic Surgery, University of Malaya Medical Centre, using a Young’s modulus of 200 GPa and a Poisson’s ratio of 0.28. The bone cement was modelled while the THA femur was reconstructed by inserting stem prosthesis into the femoral canal. The effects of different proximal cement lengths in THA were investigated by analyzing the stress distribution and displacement of the THA model during walking and stair climbing.

Cemented THA; Finite element analysis; Proximal cementation; Total hip arthroplasty

The number of patients who have undergone total hip arthroplasty (THA) is continually increasing. The most common indicators for primary THA are osteoarthritis, particularly for older patients (aged 75 years and over), avascular necrosis, rheumatoid arthritis, developmental dysplasia of the hip, and osteoporosis (Amanatullah et al., 2010). Normally, these patients can barely walk and experience continuous pain in their daily lives. THA is believed to help increase quality of life and improve joint function, allowing older patients to function normally and younger patients to resume sports activities. During THA, a surgeon makes an incision over the head and proximal neck of the femur and removes layers of the hip socket. Then, a metal ball and stem are inserted in the femur and a plastic socket is placed in the enlarged pelvis cup. To obtain successful results, these components must be fixed firmly to the bone, either with polymethylmethacrylate cement or a cementless fixation, via bone ingrowth into a porous surface, resulting in biologic fixation (Choi, 2015). 

THA is believed to be the greatest advancement in surgery in the second half of the 20^th century. In the late 1940s, researchers experimented with and developed many different materials, surgical techniques, fixation methods, and implant design. Sir John Charnley attempted to design an artificial hip joint using biomechanical principles of human hip joint function (Abdullah et al., 2010). The outcome yielded very low friction bearing surfaces that helped reduce friction and wear rates, resulting delayed aseptic loosening. Several aspects marked the prosthesis as successful, namely longevity, ease of implantation, and revisability.

Many factors need to be focused on, such as implant design, materials, and fixation. The fixation method can be cemented or uncemented. Cemented methods were widely used in early procedures and have gained popularity since being introduced by John Charnley in 1972; they have been continuously improved throughout the decades (Abdullah et al., 2010). They use polymethylmethacrylate (PMMA), or bone cement, to affix the prosthesis to the bone. Prior to surgery, the femoral canal is injected with bone cement to secure the prosthesis in its ideal position.

The type of fixation method used depends on the patient’s age. For example, cementless fixation is favored for young and active patients. Though many studies have been conducted on cemented and cementless hip arthroplasty (Abdullah et al., 2017; Zhang et al., 2017; Todo, 2018), no studies have specifically focused on proximal cementation fixation in fully cemented implants and the effects of bone cement length. Einsiedel et al. (2008) reported the advantages of proximal hip stem fixation based on their findings using the new Z-shaft implant. It was a partially cemented stem, known as a hybrid model, with proximal cementation and cranial press fit. Similar studies describing the performance of proximal fixation referred to the hybrid stem model (Pennington et al., 2013; Samra and Paliobeis, 2015; Valle et al., 2016; Wangen et al., 2017; Zhang et al., 2017; Jonas et al., 2019; Nawfal et al., 2020). In this study, the proposed proximal fixation was applied to the fully cemented implant model. We expect to improve existing cemented stem fixation by minimizing the usage of bone cement. Hence, this study will predict the optimum cement length and the effects of different cement lengths in cemented hip arthroplasty by analyzing stress distribution and displacement.

The stress–strain distribution and displacement of different cemented THAs during stair climbing and walking were successfully analyzed using FEA. The results showed that the von Mises stress value did not exceed the yield strength, which were 115 MPa, 205 MPa, and 29 MPa, for femoral bone, stem prosthesis and bone cement, respectively. Yield strength is the stress at which a material begins to deform plastically, while yield point is when non-linear deformation begins. Hence, no models were permanently deformed. The highest displacement values for the cement mantle were 3.376 ?m and 3.278 ?m for stair climbing and walking activities, respectively. The total displacement increment of the stem and cement mantle with increasing proximal cement cut off suggests the risk of implant loosening at higher cement cut offs.

    This research was supported by Universiti Teknologi MARA, UiTM under Grant No. 600-IRMI/PERDANA 5/3 BESTARI (103/2018). We thank and acknowledge the Ministry of Higher Education in Malaysia, our colleagues from the Department of Orthopaedic Surgery, University of Malaya Medical Centre, and the UiTM Penang Branch for their computational software facilities and for providing insight and expertise in the research work.

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