The design, manufacture and analysis of a new implant for fracture fixation in human and veterinary orthopaedic surgery: the bone fastenerod

Fracture fixation in humans and animals has troubled surgeons and scientists since first it was attempted right up to the present day. At every milestone of achievement in the understanding and practice of fracture repair there has remained a significant problem left unresolved. Of paramount concern is the preservation of blood vessels and soft tissues, avoidance of stress shielding and concentration, promotion. of bone healing and a rapid return to function. However, matching these principles with the variables of degree and site of fracture/injury, age, size and status of patient, environmental and surgical factors is complex and difficult. To be able to attempt to allow a surgeon to make decisions about every case, knowing that the implant choice does not constrain him but offers flexibility to aim for the ideal fixation for each case, the system must be modular. The objectives were to produce an implant system that would satisfy the most up to date principles of fracture repair through design optimization, mechanical evaluation and testing for specific fracture types. The design was called the bone fastenerod following the optimization and analysis procedures to indicate the origins of its basic formation. To begin with, the design of the fastenerod had to be optimized and this was achieved using bench testing, initially of selected designs followed by finite element analysis, which allowed a greater number of designs to be processed. Once the optimum design had been found the process of manufacture had to be selected and various possible methods of manufacture were examined until the one most suitable was determined. To analyze the fastenerod, the current industry standard implants that are used in the same clinical type settings were chosen for comparative testing. Testing was performed using static and cyclic loading to failure with wood samples in four point, tensile, side, axial and torsional loading Specific fractures in dog, cat and horse bones were created and repaired using the fastenerod versus the best method currently available and tested in three point, tensile, axial, static loading to failure. Also, specific fractures were created in human mechanical bones and tested using axial, cyclic and four point bending again, comparing the fastenerod w ith the best technology available. The analysis revealed that in static loading the fastenerod was comparable to the industry standards for small implants but not comparable with the large human implants in the specific cases selected However, in the case of the cyclic loading to Mure , the fastenerod performed better than the plate system of similar size, with the ultimate load to failure being higher and no stress concentration leading to implant fracture or failure. Thus, the modular system of the bone fastenerod could now claim to provide fixation that could be flexible, less invasive and destructive to tissues, capable o f greater choice o f screw placement and stiff to level of choice whilst avoiding stress. concentration and shielding On the basis o f this analysis, the fastenerod system can proceed to fu ll c lin ica l trial

A Biomechanical Analysis of One-Third Tubular Plates for the Treatment of Benign Lesions in the Distal Femur

The purpose of this study was to evaluate the use of one-third tubular plates for the treatment of benign defects in the medial distal metaphysis of the femur. Benign cysts are a common occurrence in long bones, and are of concern in load-bearing bones, such as the tibia and femur. These space-occupying growths are removed by curettage of the affected region. Numerous post-curettage management options have been described in the literature, which generally include filling the defect with either synthetic or biological materials. Unfortunately, complications, such as infectious disease transmission, thermal injury, and a robust inflammatory have all been reported in the literature. In response to these concerns, a number of studies reported successful healing of benign cortical defects in long bones with no augmentation after curettage, however, the lack of structural support results in an increased risk of fracture through the defect site. Therefore, it is advantageous to investigate a treatment option that adds structural support to the defect site and permits the use of osteoconductive and osteoinductive materials within the bone cavity. The purpose of this thesis was threefold: First, a quasi-static experimental comparison of intact and cortical defect specimens was conducted to determine the structural consequences incurred by the introduction of a 15 mm cortical defect under isolated axial and torsional loads. Second, an experimental combined axial/torsional fatigue analysis was employed to further analyze the behavior of the defect specimens, and to determine the structural stiffness regained by the addition of a one-third tubular plate. Third, a numerical approach was used to consider the structural consequences of varying sized defects under isolated and combined quasi-static axial and torsional loading, and to further analyze the results of adding the plate to the defect specimens. This study revealed that a one-third tubular plate might be a clinically viable option for structural support of small cortical defects in the distal femur. Furthermore, the loss in stiffness by the defect is exacerbated under combined axial/torsional loading. This is a more physiologically relevant loading mode and may provide more clinically useful results

Mechanical Behavior of Elastic Self-Locking Nails for Intramedullary Fracture Fixation: A Numerical Analysis of Innovative Nail Designs

Intramedullary nails constitute a viable alternative to extramedullary fixation devices; their use is growing in recent years, especially with reference to self-locking nails. Different designs are available, and it is not trivial to foresee the respective in vivo performances and to provide clinical indications in relation to the type of bone and fracture. In this work a numerical methodology was set up and validated in order to compare the mechanical behavior of two new nailing device concepts with one already used in clinic. In detail, three different nails were studied: (1) the Marchetti-Vicenzi's nail (MV1), (2) a revised concept of this device (MV2), and (3) a new Terzini-Putame's nail (TP) concept. Firstly, the mechanical behavior of the MV1 device was assessed through experimental loading tests employing a 3D-printed component aimed at reproducing the bone geometry inside which the device is implanted. In the next step, the respective numerical model was created, based on a multibody approach including flexible parts, and this model was validated against the previously obtained experimental results. Finally, numerical models of the MV2 and TP concepts were implemented and compared with the MV1 nail, focusing the attention on the response of all devices to compression, tension, bending, and torsion. A stability index (SI) was defined to quantify the mechanical stability provided to the nail-bone assembly by the elastic self-locking mechanism for the various loading conditions. In addition, results in terms of nail-bone assembly stiffness, computed from force/moment vs. displacement/rotation curves, were presented and discussed. Findings revealed that numerical models were able to provide good estimates of load vs. displacement curves. The TP nail concept proved to be able to generate a significantly higher SI (27 N for MV1 vs. 380 N for TP) and a greater stiffening action (up to a stiffness difference for bending load that ranges from 370 Nmm/° for MV1 to 1,532 Nmm/° for TP) than the other two devices which showed similar performances. On the whole, a demonstration was given of information which can be obtained from numerical simulations of expandable fixation devices

Temporal evolution of mechanical properties of skeletal tissue regeneration in rabbits. An experimental study

Various mathematical models represent the effects of local mechanical environment on the regulation of skeletal regeneration. Their relevance relies on an accurate description of the evolving mechanical properties of the regenerating tissue. The object of this study was to develop an experimental model which made it possible to characterize the temporal evolution of the structural and mechanical properties during unloaded enchondral osteogenesis in the New Zealand rabbit, a standard animal model for studies of osteogenesis and chondrogenesis. A 25mm segment of tibial diaphysis was removed sub-periosteally from rabbits. The defect was repaired by the preserved periosteum. An external fixator was applied to prevent mechanical loading during osteogenesis. The regenerated skeletal tissues were studied by CT scan, histology and mechanical tests. The traction tests between 7 to 21 days post-surgery were done on formaldehyde-fixated tissue allowing to obtain force/displacement curves. The viscoelastic properties of the regenerating skeletal tissues were visualized throughout the repair process.Comment:

Caracterización biomecánica del hueso de oveja

El present estudi té com a propòsit l’elaboració d’una metodologia de treball que permeti descriure i caracteritzar les propietats biomecàniques de l’os d’ovella. L’ús del model animal oví és molt utilitzat en la investigació i desenvolupament de pròtesis humanes i, per aquest motiu, hi ha la necessitat de poder caracteritzar les seves propietats. Les mostres d’os són sotmeses a assajos mecànics clàssics de compressió i flexió, a tres i quatre punts, per tal de caracteritzar així el seu comportament. L’estudi de l’os com a teixit viu permetrà assentar les bases necessàries per a realitzar aquest project

Temporal analysis of mechanical properties of skeletal tissue regeneration in New Zealand rabbits. An experimental study

7International audienceVarious mathematical models represent the effects of local mechanical environment on the regulation of skeletal regeneration. Their relevance relies on an accurate description of the evolving mechanical properties of the regenerating tissue. The object of this study was to develop an experimental model which made it possible to characterize the temporal evolution of the structural and mechanical properties during unloaded enchondral osteogenesis in the New Zealand rabbit, a standard animal model for studies of osteogenesis and chondrogenesis. A 25mm segment of tibial diaphysis was removed sub-periosteally from rabbits. The defect was repaired by the preserved periosteum. An external fixator was applied to prevent mechanical loading during osteogenesis. The regenerated skeletal tissues were studied by CT scan, histology and mechanical tests. The traction tests between 7 to 21 days post-surgery were done on formaldehyde-fixated tissue allowing to obtain force/displacement curves. The viscoelastic properties of the regenerating skeletal tissues were visualized throughout the repair process

Alendronate treatment elicits a reduction in fatigue-life of canine cortical bone

Bone serves contradictory needs; bone must be strong yet light, and stiff yet flexible. At the tissue level bone material withstands cyclic loading without failing by dissipating energy via the formation and accumulation of microdamage. Proper removal of this damage in exchange for fresh tissue is vital to bone maintenance, and is achieved through a remodeling process. Imbalanced remodeling leads to osteoporotic fractures. Bisphosphonate drugs are proven to reduce fracture risk. However, the long-term effects of bisphosphonates on tissue-level properties are unknown. This study characterized the fatigue-life of cortical bone tissue after bisphosphonate treatment with alendronate (Aln). 1 1th ribs from 36 skeletally mature female beagles (1-2 years of age) treated daily with either a vehicle control (Cont, 1mL/kg saline) or Aln (0.2 or 1.0 mg/kg) for 3 years were evaluated. From both medial and lateral cortices, 1-6 cortical bone beams of uniform rectangular cross-section (0.5 x 1.5 mm) and length (10 - 12 mm) were prepared. A total of 90 bone beams were mechanically loaded in 4-point bending at specific stress amplitudes, 45- 85 MPa, applied sinusoidally at 2 Hz until fracture or 250,000 cycles. Compared to control, Aln 1.0 beams exhibited significantly lower initial stiffness (15%) and cycles to failure (\u3e3-fold, p\u3c0.05). While control exhibited increased loss of stiffness as a function of increasing stress amplitude, this was not observed with Aln treatment. This first fatigue study of bisphosphonate-treated bone suggests mechanisms behind the atypical cortical bone fracture patterns that have been observed clinically in a subset of patients on long-term bisphosphonate treatment

A novel concept of non-metallic orthopedic implants for load-bearing applications

Metallic implants have remained the state of the art for skeletal reconstruction for over a hundred years. However, the excessive stiffness of metallic implants can lead to unphysiological direct bone healing and bone resorption in both humans and animals. Fiber reinforced composites (FRC) were proposed as alternative to metals in the “less rigid fixation” concept and were successfully approved in animal models and in clinical trials in humans as early as in 1970th and 1980th. However, the use of FRC implants is still limited due to such drawbacks as the lack of contourability, which is desirable in many applications, and high manufacturing costs. The present dissertation addresses the limitations of FRC implants and suggests a novel concept of non-metallic load-bearing implants. The research further develops the “less rigid fixation” concept and complements it with the application of modern additive manufacturing and composite technologies, such as 3D printing and tailored fiber placement (TFP). The novel concept assumes that the design of the implants does not simply copy the existing conventional metallic counterparts but is initially aimed at the specified fabrication methods to reduce the costs and improve the tailorability and scalability for different clinical conditions and patients. The concept allows application of both biostable and bioresorbable polymeric materials, can be implemented in various designs and can be combined with bioactive agents to stimulate bone growth and decrease the rate of complications related to infections. The dissertation includes four multidisciplinary studies in which the concept was described, and the basic aspects of that were investigated. Two different clinical problems, frequently occurred in veterinary practice, were addressed. Two prototype designs, an FRC fracture fixation plate, made by TFP and intended for the treatment of antebrachial fractures in toy-breed dogs, and a 3D-printed bioresorbable bioactive tibial tuberosity advancement implant for the treatment of cranial cruciate ligament disease in large dogs, were proposed based on the findings of the research. It is expected that the novel implants can improve the ossification and decrease the complications rate in animals. In addition, the present research serves as the first step towards the anticipated implementation of the novel concept in implants for humans.Uudenlainen konsepti metallittomiksi ortopedisiksi implanteiksi kuormaa kantaviin sovelluksiin Luuston korjauksessa on yli sadan vuoden ajan käytetty pääosin metallisia implantteja. Niiden jäykkyys voi kuitenkin johtaa epäfysiologiseen suoraan luunmuodostukseen ja luun resorptioon sekä ihmisillä että eläimillä. Kuitulujitettuja komposiittimateriaaleja (fiber reinforced composite, FRC) on 1970- ja 1980-luvuilta lähtien onnistuneesti kokeiltu eläinkokeissa ja kliinisissä kokeissa vähemmän jäykkänä vaihtoehtona metalleille. FRC-implanttien käyttö on toistaiseksi ollut rajallista, sillä valmistuksen jälkeen ne eivät ole muovattavissa, mikä olisi suotava ominaisuus monessa käytännön sovellutuksessa, ja niiden valmistus on kallista. Tässä väitöskirjassa paneudutaan FRC-implanttien puutteisiin ja ehdotetaan uudenlaista konseptia metallittomiksi kuormaa kantaviksi implanteiksi. Tutkimuksessa kehitettiin vähemmän jäykkää luufiksaatiota moderneilla 3D-tulostus- ja räätälöity kuitusijoitus (tailored fiber placement, TFP) -tekniikoilla. Implantit on varta vasten valituille valmistustekniikoille suunniteltu, eivätkä pelkkiä kopioita metallisista vastineistaan. Tarkoituksena on vähentää valmistuksen kustannuksia ja parantaa implanttien muokattavuutta erilaisia kliinisiä tilanteita ja potilaita varten. Konsepti mahdollistaa sekä biohajoavien että biohajoamattomien implanttien valmistuksen eri muodoissa, ja siihen voidaan yhdistää bioaktiivisia yhdisteitä stimuloimaan luun kasvua ja vähentämään infektioista johtuvia komplikaatioita. Väitöskirjan neljässä poikkitieteellisessä osatyössä konsepti kuvaillaan ja sen perusominaisuuksia tutkitaan. Implanttiprototyypit kehitettiin kahteen yleiseen eläinlääketieteelliseen kliiniseen ongelmaan: TFP-tekniikalla valmistettu FRCfiksaatiolevy radiuksen ja ulnan murtumien hoitoon pienikokoisilla koirilla, ja isojen koirien polven eturistisidevaurioiden hoitoon tarkoitettu 3D-tulostettu bioaktiivinen ja bioresorboituva implantti sääriluun kyhmyn eteenpäin siirtämiseksi. Oletuksena on, että implantit parantavat luutumista ja vähentävät komplikaatioriskiä eläimillä. Lisäksi tämä tutkimus on ensimmäinen askel tämän uudenlaisen konseptin käyttöön ihmisillä