Comparison of constrained and unconstrained ICR types of artificial disc replacements using FE model
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$524 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) Mean axial pullout strength found to be 232 N (range 60-600 N) in group 1, 452N (range 60-1125N) in group 2 and 367N (range 112~}40N) in group 3. The paired Student t-test demonstrated a significant difference between pullout strength of group 1 and group 2 (p=0.0300). Between pullout strength of group 1 and group 3 and between group 2 and group 3 no significant difference was seen. A 1.9 times higher pullout strength with kyphoplasty augmentation of osteoporotic vertebrae compared with the pullout strength of nonaugmented vertebrae was obtained. Implantation of pedicle screws in cured cement is a sufficient method. With this method a 1.6 times higher pullout strength than in nonaugmented vertebrae was found. 5377 We-Th, no. 55 (P59) Comparison of constrained and unconstrained ICR types of artificial disc replacements using FE model Y.E. Kim, S.S. Yun, S.T. Kim. Dept. ef Mechanical Engineering, Dankeek University, Seoul, South Korea Although several artificial disc designs have been developed for the treatment of discogenic low back pain, biomechanical changes with its implantation were rarely studied. To evaluate the effect of artificial disc implantation on the biome- chanics of functional spinal unit, a nonlinear three-dimensional finite element model of L4-L5 was developed with 1 mm CT scan data. Biomechanical anal- ysis was performed for two different types of artificial disc having constrained and unconstrained instant center of rotation(ICR), ProDisc and SB Charit6 III model. The implanted model predictions were compared with that of intact model. Angular motion of vertebral body, forces on the spinal ligaments and facet joint, and stress distribution of vertebral endplate for flexion-extension, lateral bending, and axial rotation with a compressive preload of 400 N were compared. The implanted model showed increased flexion-extension range of motion compared to that of intact model. Under 6 Nm moment, the range of motion were 140%, 170% and 200% of intact in SB Charit6 III model and 133%, 137%, and 138% in ProDisc model. The increased stress distribution on vertebral endplate for implanted cases could be able to explain the heterotopic ossification around vertebral body in clinical observation. Facet contact forces and flexion motion were sensitive to the antero-posterior location of artificial disc especially for constrained ICR type. As a result of this study, it is obvious that implanted segment with artificial disc suffers from increased motion and stress that can result in accelerated degenerated change of surrounding structure. Unconstrained ICR model showed increased in motion but less stress in the implanted segment than constrained model. 4.5 Endoprosthetics of the Shoulder, Elbow, Foot and Ankle 6004 We-Th, no. 56 (P59) Correlation of BMD of the glenoid bone with the pullout strength of Mitek GII anchors loaded with Orthocord TM suture R. Mim'ar 1, R.M. Hall 1,2, D. Limb 2. 1School ef Mechanical Engineering, University of Leeds, Leeds, UK, 2Academic Unit of Orthopaedic Surgery, St James's University Hospital, Leeds, UK Suture anchors, such as the Mitek GII, are effective implants for soft tissue reattachment to bone and have been widely studied in the context of shoulder surgery. Clinical studies have shown the Mitek GII to be effective for instability repairs whilst in vitro pull out tests have shown that it is often the suture material that fails under extreme loading conditions. This study investigated the relationship between pullout strength and BMD when using GII anchors loaded TM with Orthocord TM (Depuy, USA) sutures. Orthocord sutures comprise a new material that has been shown to have superior strength characteristics but as yet no clinical studies have confirmed that this has any in vivo advantage. Eight embalmed cadaveric specimens were used with the procedure to fix the anchors in place being undertaken by one experienced shoulder surgeon (DL). The structural characteristics of the anchor/suture fixation were determined using a tensile testing machine whilst the local BMD at the anchor site was determined using a microCT. Six procedures failed within the suture material (189±25N) whilst the remaining 25 failed by anchor pullout (126±40N). For the procedures that failed by loss of integrity of the anchor there was observed a significant correlation between pullout strength and the local BMD (rpearso n =0.56, P=0.004). Comparison with results for anchor pullout performed elsewhere indicates that the use of embalmed cadaveric specimens for these types of tensile tests may alter significantly the absolute values of the pullout strength observed [1,2]. However, it is likely that the rank order and therefore the correlation between BMD and strength remains valid and that tissue quality is an important determinant of anchor strength. Further work on fresh cadaveric tissue is on going. References [1] Rupp S., et al. Am J Sports Med. 2002; 30: 239-247. [2] Leedle B.P., et al. Arthoscopy 2005; 21: 81. Poster Presentations 4736 We-Th, no. 57 (P59) Net torsional moments with unconstrained ankle arthroplasty: measurements, models and consequences A. Stacoff, M. Dettwyler, R. List, E. StiJssi. Laboratory for Biomechanics, D-MA VT[, ETH Zurich, Zurich, Switzerland Introduction: Today's unconstrained ankle arthroplasties have a center com- ponent consisting of a low friction high-density polyethylene (inlay) that basi- cally does not transmit shear forces between the talus and the tibia [Pappas et al., 2005]. The healthy human ankle, however, transmits moments [Burge et al., 1986] during the stance phase in walking. Thus, the hypothesis was tested that patients with ankle arthroplasties show decreased tibial torsion moments compared to healthy controls. Methods: The patient group (n = 10) with a BuecheI-Pappas arthroplasty was fully rehabilitated; the control group consisted also of 10 subjects. Motion capturing was performed (Vicon) with the foot and shank each redundantly equipped with 5 markers. The ankle joint was modeled as a ball-and-socket joint with a person specific ankle joint center computed from loaded plantar- dorsiflexion and eversion-inversion movements using an optimization routine. Force data was collected (Kistler 9281B) and tibia torsional moments were computed; mechanical modeling allowed an approximation of the stiffness around the tibial axis [Dettwyler, 2005]. Results and Discussion: The present study is the first to show that, compared to the controls, patients with an unconstrained ankle arthroplasty are capable of producing a similar amount of net tibial torsional moment (study hypothesis rejected). It was concluded that muscles and ligaments at the ankle are subjected to a higher level of load in patients. Ankle ligaments may provide resistance of about 2 Nm/° for foot internal and about 2.5 Nm/° for foot external rotation. Modeling of the loaded articular cartilage of a healthy ankle joint revealed a rotational stiffness about the vertical axis of about 1-2 Nm/° for the peak compression forces [Dettwyler, 2005] in contrast to the arthroplastic joint where this effect is not present. Summary: In contrast to the expectation, patients with ankle arthroplasties produce normal net tibial moments by an altered load transmitting strategy at the ankle, a finding which needs to be considered in future ankle implant designs. Acknowledgements: P. Rippstein, Schulthess Clinic, Zurich; ISB Dissertation Grant, 2004. References Burge et al. (1986). Foot and Ankle 7(1): 10-17. Dettwyler M.T. (2005). Dissertation ETH Z~irich. Pappas M. and Buechel R (2005). www.endotec.com. 7688 We-Th, no. 58 (P59) High-tech or solving the needs in developing countries U. Lanza, C. MiJller-Karger, C. Graciano. Universidad Sim6n Bolivar, Department of Mechanical Engineering, Caracas, Venezuela Current advances in medicine and technology have lead to the development of very sophisticated techniques in designing and manufacturing prosthetic devices. High technology is available in some countries and at very high cost. However, in developing countries there are still basic needs to be satisfied, where patients due to economical limitations are restricted to use prosthesis made of simple techniques. It means that the construction of low cost prosthesis is an intricate challenge. This paper presents the process of designing and manufacturing a transtibial prosthesis for a 14 year old patient in Venezuela. The subject of this investigation presented a residual limb and due to the misalignment and not appropriated size of a previous prosthesis the patient was also suffering of Genu Recurvatum and Genu Valgum. Therefore, this new prosthesis was designed to solve several problems. TAC were taken to the patient to determine accurately shape and length of residual limb, and to perform a finite element analysis of the contact between residual limb and socket. According to the measures taken from a bone model reconstruction using TAC images, the prosthetic adapter, and the socket were designed and manufactured. The socket was thermoformed by drape forming using PP copolymer in a homemade vacuum system recovered from a dismantled compressor, working in reverse. The adapter that connects the socket with the artificial foot was manufactured using Nylon 6 6, connecting the parts with a clamping device. All the prosthesis was fabricated using material found locally and inexpensive. The patient is very satisfied with the new prosthesis, which shows how the will and the motivation of the participants are more important, in the development of prosthetic devices, than the lack of high-tech.
Transcript of Comparison of constrained and unconstrained ICR types of artificial disc replacements using FE model
$524 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)
Mean axial pullout strength found to be 232 N (range 60-600 N) in group 1, 452N (range 60-1125N) in group 2 and 367N (range 112~}40N) in group 3. The paired Student t-test demonstrated a significant difference between pullout strength of group 1 and group 2 (p=0.0300). Between pullout strength of group 1 and group 3 and between group 2 and group 3 no significant difference was seen. A 1.9 times higher pullout strength with kyphoplasty augmentation of osteoporotic vertebrae compared with the pullout strength of nonaugmented vertebrae was obtained. Implantation of pedicle screws in cured cement is a sufficient method. With this method a 1.6 times higher pullout strength than in nonaugmented vertebrae was found.
5377 We-Th, no. 55 (P59) Comparison of constrained and unconstrained ICR types of art i f icial disc replacements using FE model Y.E. Kim, S.S. Yun, S.T. Kim. Dept. ef Mechanical Engineering, Dankeek University, Seoul, South Korea
Although several artificial disc designs have been developed for the treatment of discogenic low back pain, biomechanical changes with its implantation were rarely studied. To evaluate the effect of artificial disc implantation on the biome- chanics of functional spinal unit, a nonlinear three-dimensional finite element model of L4-L5 was developed with 1 mm CT scan data. Biomechanical anal- ysis was performed for two different types of artificial disc having constrained and unconstrained instant center of rotation(ICR), ProDisc and SB Charit6 III model. The implanted model predictions were compared with that of intact model. Angular motion of vertebral body, forces on the spinal ligaments and facet joint, and stress distribution of vertebral endplate for flexion-extension, lateral bending, and axial rotation with a compressive preload of 400 N were compared. The implanted model showed increased flexion-extension range of motion compared to that of intact model. Under 6 Nm moment, the range of motion were 140%, 170% and 200% of intact in SB Charit6 III model and 133%, 137%, and 138% in ProDisc model. The increased stress distribution on vertebral endplate for implanted cases could be able to explain the heterotopic ossification around vertebral body in clinical observation. Facet contact forces and flexion motion were sensitive to the antero-posterior location of artificial disc especially for constrained ICR type. As a result of this study, it is obvious that implanted segment with artificial disc suffers from increased motion and stress that can result in accelerated degenerated change of surrounding structure. Unconstrained ICR model showed increased in motion but less stress in the implanted segment than constrained model.
4.5 Endoprosthetics of the Shoulder, Elbow, Foot and Ankle 6004 We-Th, no. 56 (P59) Correlation of BMD of the glenoid bone with the pullout strength of Mitek GII anchors loaded with Orthocord TM suture R. Mim'ar 1 , R.M. Hall 1,2, D. Limb 2. 1School ef Mechanical Engineering, University of Leeds, Leeds, UK, 2Academic Unit of Orthopaedic Surgery, St James's University Hospital, Leeds, UK
Suture anchors, such as the Mitek GII, are effective implants for soft tissue reattachment to bone and have been widely studied in the context of shoulder surgery. Clinical studies have shown the Mitek GII to be effective for instability repairs whilst in vitro pull out tests have shown that it is often the suture material that fails under extreme loading conditions. This study investigated the relationship between pullout strength and BMD when using GII anchors loaded
TM with Orthocord TM (Depuy, USA) sutures. Orthocord sutures comprise a new material that has been shown to have superior strength characteristics but as yet no clinical studies have confirmed that this has any in vivo advantage. Eight embalmed cadaveric specimens were used with the procedure to fix the anchors in place being undertaken by one experienced shoulder surgeon (DL). The structural characteristics of the anchor/suture fixation were determined using a tensile testing machine whilst the local BMD at the anchor site was determined using a microCT. Six procedures failed within the suture material ( 1 8 9 ± 2 5 N ) whilst the remaining 25 failed by anchor pullout (126±40N) . For the procedures that failed by loss of integrity of the anchor there was observed a significant correlation between pullout strength and the local BMD (rpearso n =0 .56 , P=0.004). Comparison with results for anchor pullout performed elsewhere indicates that the use of embalmed cadaveric specimens for these types of tensile tests may alter significantly the absolute values of the pullout strength observed [1,2]. However, it is likely that the rank order and therefore the correlation between BMD and strength remains valid and that tissue quality is an important determinant of anchor strength. Further work on fresh cadaveric tissue is on going.
References [1] Rupp S., et al. Am J Sports Med. 2002; 30: 239-247. [2] Leedle B.P., et al. Arthoscopy 2005; 21: 81.
Poster Presentations
4736 We-Th, no. 57 (P59) Net torsional moments with unconstrained ankle arthroplasty: measurements, models and consequences A. Stacoff, M. Dettwyler, R. List, E. StiJssi. Laboratory for Biomechanics, D-MA VT[, ETH Zurich, Zurich, Switzerland
Introduction: Today's unconstrained ankle arthroplasties have a center com- ponent consisting of a low friction high-density polyethylene (inlay) that basi- cally does not transmit shear forces between the talus and the tibia [Pappas et al., 2005]. The healthy human ankle, however, transmits moments [Burge et al., 1986] during the stance phase in walking. Thus, the hypothesis was tested that patients with ankle arthroplasties show decreased tibial torsion moments compared to healthy controls. Methods: The patient group (n = 10) with a BuecheI-Pappas arthroplasty was fully rehabilitated; the control group consisted also of 10 subjects. Motion capturing was performed (Vicon) with the foot and shank each redundantly equipped with 5 markers. The ankle joint was modeled as a ball-and-socket joint with a person specific ankle joint center computed from loaded plantar- dorsiflexion and eversion-inversion movements using an optimization routine. Force data was collected (Kistler 9281B) and tibia torsional moments were computed; mechanical modeling allowed an approximation of the stiffness around the tibial axis [Dettwyler, 2005]. Results and Discussion: The present study is the first to show that, compared to the controls, patients with an unconstrained ankle arthroplasty are capable of producing a similar amount of net tibial torsional moment (study hypothesis rejected). It was concluded that muscles and ligaments at the ankle are subjected to a higher level of load in patients. Ankle ligaments may provide resistance of about 2 Nm/° for foot internal and about 2.5 Nm/° for foot external rotation. Modeling of the loaded articular cartilage of a healthy ankle joint revealed a rotational stiffness about the vertical axis of about 1-2 Nm/° for the peak compression forces [Dettwyler, 2005] in contrast to the arthroplastic joint where this effect is not present. Summary: In contrast to the expectation, patients with ankle arthroplasties produce normal net tibial moments by an altered load transmitting strategy at the ankle, a finding which needs to be considered in future ankle implant designs. Acknowledgements: P. Rippstein, Schulthess Clinic, Zurich; ISB Dissertation Grant, 2004.
References Burge et al. (1986). Foot and Ankle 7(1): 10-17. Dettwyler M.T. (2005). Dissertation ETH Z~irich. Pappas M. and Buechel R (2005). www.endotec.com.
7688 We-Th, no. 58 (P59) High-tech or solving the needs in developing countries U. Lanza, C. MiJller-Karger, C. Graciano. Universidad Sim6n Bolivar, Department of Mechanical Engineering, Caracas, Venezuela
Current advances in medicine and technology have lead to the development of very sophisticated techniques in designing and manufacturing prosthetic devices. High technology is available in some countries and at very high cost. However, in developing countries there are still basic needs to be satisfied, where patients due to economical limitations are restricted to use prosthesis made of simple techniques. It means that the construction of low cost prosthesis is an intricate challenge. This paper presents the process of designing and manufacturing a transtibial prosthesis for a 14 year old patient in Venezuela. The subject of this investigation presented a residual limb and due to the misalignment and not appropriated size of a previous prosthesis the patient was also suffering of Genu Recurvatum and Genu Valgum. Therefore, this new prosthesis was designed to solve several problems. TAC were taken to the patient to determine accurately shape and length of residual limb, and to perform a finite element analysis of the contact between residual limb and socket. According to the measures taken from a bone model reconstruction using TAC images, the prosthetic adapter, and the socket were designed and manufactured. The socket was thermoformed by drape forming using PP copolymer in a homemade vacuum system recovered from a dismantled compressor, working in reverse. The adapter that connects the socket with the artificial foot was manufactured using Nylon 6 6, connecting the parts with a clamping device. All the prosthesis was fabricated using material found locally and inexpensive. The patient is very satisfied with the new prosthesis, which shows how the will and the motivation of the participants are more important, in the development of prosthetic devices, than the lack of high-tech.
