Evaluation of large acetabular bone defects

Schierjott, Ronja Alissa (2021): Evaluation of large acetabular bone defects: conception and implementation of suitable biomechanical test methods. Dissertation, LMU München: Medizinische Fakultät

Abstract

Objectives The objectives of this dissertation were to assess which acetabular bone defects associated with revision surgery exist and to analyze these defects in a quantitative way. Furthermore, the aim was to develop an acetabular bone defect model to assess the influence of bone defect and bone defect filling on the primary stability of a press-fit cup, and to ascertain whether the type of filling material influences primary stability, i.e. if synthetic BGS can achieve primary stability comparable to the gold standard bone chips. Background Aseptic loosening is one of the main reasons for revision in THA. This is most commonly caused by particle-induced osteolysis but can, besides others, also be induced by excessive relative motion between implant and bone, which leads to the formation of fibrous tissue and further loosening, eventually resulting in revision surgery. Revision surgery is often associated with acetabular bone defects due to periprosthetic osteolysis, which make the following implant fixation difficult. In order to categorize the defects, numerous classification schemes have been published. However, most of them are rather descriptive and hence difficult to transfer to implant development or pre-clinical testing. To treat the defects, numerous implant systems and treatment strategies exist, besides others the impaction bone grafting technique (IBG), whereby bone chips are compacted into a defect and combined with a cup, reinforcement ring or reconstruction cage. However, the bone chips have some disadvantages such as limited supply and remaining infection risk. Synthetic bone graft substitutes (BGS) may represent an attractive alternative, but their potential to reestablish primary stability in combination with press-fit cups has yet hardly been assessed. Materials and methods First, 50 computed tomography (CT) data sets of pelvises with acetabular bone defects were analyzed and quantified using six parameters. In order to do so, virtual 3D models of the defect pelvises were created, and the corresponding “native” situation was reconstructed via a statistical shape model (SSM) for each pelvis individually. Based on these models, parameters such as bone volume loss and new bone formation in specific areas of the acetabulum could be assessed. On the basis of the quantitative defect analysis, a representative defect was chosen in consultation with the clinical advisors for the following pre-clinical tests. In order to later on enable defect implementation with simple tools in human donor specimens, its geometry was simplified. For this purpose, nine reaming procedures with hemispherical reamers were defined in order to reproduce the defect with its main characteristics. From the resulting “complete defect” (defect increment 4), three additional, less severe bone defect increments were derived by exclusion of specific reaming procedures (defect increments 1-3). The resulting platform concept was developed to test different revision treatment strategies in defects with increasing severity in the future. A surrogate acetabular model (AM) made of polyurethane foam was developed, in which the defect (increment 1: Mainly medial contained defect with damage of the posterior-inferior rim) was implemented. Within this dissertation, a total of six test groups were investigated concerning their primary stability: Primary situation (AM without defect, treated with press-fit cup), defect situation (AM with defect, treated with press-fit cup), and four different types of defect filling (AM with defect, treated with press-fit cup and bone chips / three different types of BGS). The specimens were loaded dynamically and primary stability in terms of relative motion between cup and AM (inducible displacement and migration) were assessed using an optical measurement system. Results The defect analysis showed a large variation of values in all analyzed parameters. However, correlations between single analysis parameters could be observed and first defect groups could be established. The representative defect chosen for the pre-clinical tests could be reproduced in a simplified way using several reaming procedures, whereby the simplified defect showed a total volume conformity of >99% with the original defect, as well as a comparable geometry. The defect implemented in publication 2 (increment 1: Mainly medial contained defect with damage of the posterior-inferior rim) led to an increase in relative motion compared to the primary situation without defect (1.9-fold increase of inducible displacement and 8.2-fold increase in migration). By filling the defect, this could be reduced again to 1.1-fold increase and 2.4-fold increase in comparison with the primary situation, respectively. Within the comparison of bone chips with two BGS in publication 3 (bioactive glass in PEG and β-TCP tetrapods in collagen) it was seen that the β-TCP tetrapods in collagen matrix showed a behavior comparable to bone chips, i.e. comparable relative motions and could hence represent an attractive alternative to the gold standard bone chips. Discussion and outlook Within this dissertation, acetabular bone defects could successfully be quantified, and one representative defect was transferred into a pre-clinical testing model. In the future, the defect analysis method could potentially be used to establish a quantitative, impartial, reproducible defect classification. In order to do so, an even larger number of defects should be analyzed, also in association with the chosen defect treatment and clinical (long-term) treatment success. In the performed in vitro tests, the negative influence of a defect on primary stability of a press-fit cup could be shown, as well as the potential of certain BGS to reestablish primary stability and hence to potentially substitute bone chips in the future. Benefits of the BGS would be the reproducible mechanical characteristics, the reduced infection risk and the improved availability. However, it is important to point out that the ability of specific BGS to reestablish primary stability was so far assessed in one specific defect and the results may be different in another defect type. Hence, primary stability should further be investigated in additional and more severe defects to obtain additional information about potential limitations of the material or prerequisites for its application. Moreover, it should be assessed whether the durability and the potential for osseointegration (pore structure and surface properties to enable bone ingrowth) of the BGS are sufficient to substitute bone chips, as these factors also influence the clinical long-term success of the defect treatment.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Large acetabular bone defects, quantitative assessment, bone graft substitutes, primary stability, relative motion measurement
• Themengebiete: 600 Technik, Medizin, angewandte Wissenschaften ; 600 Technik, Medizin, angewandte Wissenschaften > 610 Medizin und Gesundheit
• Fakultäten: Medizinische Fakultät
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 2. März 2021
• 1. Bericht­erstatter:in: Grupp, Thomas