Performance of Biodegradable Polymers used in Mechanically Loaded Implants

Lonnie Ulrich Andersen

Research output: Book/ReportPh.D. thesisResearch

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Total hip arthroplasty is the most common surgical procedure performed in the orthopaedic field, and it is considered the cornerstone in the treatment of osteoarthritis in the hip joint. It has been estimated that between 0.5-1% of the population in most Western Countries requires a total hip arthroplasty at some point in their life. The weakening of the natural stabilizers in the hip joint during the surgery, makes the patients susceptible to dislocation, in the first 3-6 months after the surgery. Studies show that approximately 3-5% of patients experience a dislocation at some stage. The factors predisposing for early dislocation have not been completely established, making it difficult to take successful preventative measures. The objective of this PhD thesis was to design an implantable, biodegradable device to guard against these dislocations.
The hip dislocation preventer should allow for easy adaptation, and mounting onto most types of hip implants, without changing the basic design of the present implant. The objective is to have a structure, which will put a restrain on the artificial hip implant as it moves into the extreme positions associated with dislocation, without further affecting the normal movement relative to the hip implant. Therefore, the stress strain profile of the device, would have to include an initial strain region, where the stress remain low, after which the stress should increase rapidly as the devise locks, preventing the dislocation. To achieve this, the design of the hip dislocation preventer should be a cone shaped mesh, encapsulating the hip implant.
Using the the basic geometry of the hip implant, a simple model was developed, describing the most common movement pattern associated with hip dislocation. The requirements concerning the initial strain region, was determined from the model. The analysis was done using two different attachment solutions, and two different locking scenarios. The results showed, that the initial strain region of the hip dislocation preventer would have to be at least 30%, and that the mesh should be able to withstand loads between 1700 N and 5000 N. Furthermore, the analysis show, that if the devise is designed to allowing the hip some degree of subluxation, the range of movement of the hip would be increased by approximately 15-19°, relative to the solution without subluxation. However, this also increases the requirements of the initial strain region by 8-13%. The conclusion to the requirement analysis, was that both attachment solutions proposed in the project, are possible in theory, depending on the distance of the mounting point on the modified acetabular cup.
The hip dislocation preventer should work as a restrictive force during the first 3-6 months, when the joint is most vulnerable to dislocations. After this period, it should slowly degrade, enabling the joint to become stronger. From the results found in the literature, poly-L-lactic acid (L-PLA) was chosen as a suitable material. In order to characterize the material, L-PLA yarns were degraded in phosphate buffered saline for 6 months. Each month, a set of test consisting of, uniaxial tensile tests, at two different deformation rates, stress relaxation tests, and creep tests were performed. The uniaxial tensile test generally show very little change in the elastic modulus, yield stress, and mean strain at break, during the first 5 to 6 months. A significant drop in the elastic modulus was observed between month 5 and 6 at both loading rates, which corresponds well with the degradation period of L-PLA. A larger number of experiments, and a longer degradation period is required, in order to determine if the small fluctuations in the properties, is a general property of the material. The stress at break was found to gradually decrease during the 6 months, and the deformations rate was found to have a significant effect on the yield stress and the stress at break, but not on the elastic modulus and strain at break. Both the stress relaxation, and the creep test show that there is a change in the material in the initial degradation period, and the results could indicate, that the material experiences less relaxation and creep, a month after the in vitro degradation period. However, there is a large variance in the data, and more tests are needed to determine if this observation is correct. The overall conclusion of the material tests of the L-PLA yarns is, that the six month degradation in vitro, did not effect the tensile properties in a way that would significantly effect their functionality. The L-PLA yarns would therefore maintain their integrity during the critical period, where the hip dislocation preventer should be functional.
The tensile properties of the L-PLA yarns were used to analyse, how a plain weave mesh would behave during different types of elongations. A model proposed by Kawabate et al in 1973 was modified, and used to analyse the problem, using different weave densities. Comparing the results found using the model, with the limits found during the strain analysis, showed that in order to maintain the full range of motion of the hip, the weave would have to be oriented in a 45° angle to the direction of deformation. From the model the initial strain region was predicted to lie between 35-40%, and the tensile force that the fabric can withstand, without going into plastic deformation was between 2000-5000 N. From the analysis and the material tests it was found that using a plain weaved LPLA mesh, the strength and flexibility needed of the hip dislocation preventer would be attainable.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages139
Publication statusPublished - 2013

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