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The effects of keel design on tibial component stability
232
The Journal of Arthroplasty Vol. 12 No. 2 February 1997
Discussion & Conclusion: The standard medial parapatellar approach devascularizes the medial branches leading to the patella. If the dissection stays within the retinacuhim and not in the muscle, it does not disturb the neural innervation to the VMC. The subvastus approach maintains the medial blood supply to the patella. If the saphenons nerve branch to the VMC exists, this approach does destroy this distal innervation to the VMC. If the subvastns approach dissects proximally about 12 cm proximal to the joint line, traction injury to the deep branch to the VMC could occur. The midvastns approach does not endanger the superficial, deep, or saphenons nerve innervation to the VMC or the vasculariza-. tion of the patella. If the dissection proceeds for more than 6 cm from the superomedial pole of the patella, the approach could interfere with the deep VMC branch. The advantages and disadvamages of all three approaches should be contemplated before TKA dissection is performed.
fracture despite the large contact stresses associated with nonconforming designs. This result indicates the complexity of oxidation processes that occur in vivo and that methods to minimize oxidation by changing sterilization methods should be evaluated closely. It is interesting to note that the lnsall/Burstein I, a conforming knee design, was also made from directly molding of 1900 resin and has a 93% survivorship at 13 years of implantation. If oxidation is a factor in the performance of total joints, directly molding with 1900 resin may provide long term performance advantages.
POSTER# 5 THE EFFECTS OF KEEL DESIGN ON TIBIAL COMPONENT STABILITY
POSTER# 4 EFFECTS OF RESIN TYPE & MANUFACTURING PROCESS ON THE OXIDATION OF UHMWPE COMPONENTS Stephen Li, PhD, New York, NY, B.D. Furman, BS, M.A. Ritter, MD Introduction: The contact stresses for total knee components of different designs have been reported to vary from 20 to 45 MPA. However, performance of these components, as measured by polyethylene damage, wear and length of implantation are not a strict function of these stresses or subsurface shear stresses. A possible explanation of this inability to predict specific performance from modeling results may lie in variations in the polyethylene quality or material properties. Further, quality and material properties may also be greatly altered by oxidation processes. Oxidation of gamma sterilized UHMWPE used in joint arthroplasty has been shown to continue for over 10 years after irradiation. This oxidative aging can cause severe embrittlement and the formation of subsurface defects and 'white bands' within the polyethylene. However, these reports did not consider the potential sensitivity of the oxidation process to different UHMWPE resin types or differences in manufacturing method. We report the analysis and comparison of oxidation and UHMWPE quality of 15 never implanted devices made from 4 different polyethylene types and 2 different manufacturing processes at 2 to 13 years post gamma irradiation. These results will be compared to the analysis of retrieved molded tihiaI components after implant times of 1 to 11 years. M a t e r i a l s a n d M e t h o d s : Never implanted c o m p o n e n t s were made from m a c h i n e d 4 1 5 G U R ( 2 ) , R C H 1 0 0 0 ( 3 ) , H y l a m e r ( 2 ) and directly m o l d e d 1900cm(8). All were gamma sterilized 2-13 years prior to examination. The components made with 1900 resin were all directly molded and all other resins were machined into the final device shape. The eleven retrieved tibial inserts were of two different, nonconforming, knee designs made from directly molded 1900 resin and were retrieved after 1-11 years implantation. Oxidation was assessed by density, FTIR and crystallinity measurements using previously reported methods. Polyethylene quality was determined by examination of cross sections of the components. Results: Never implanted components made from 415GUR, RCH1000 and Hylamer all showed very high oxidation levels (>.96g/cc) or exhibited high levels of nonconsolidated particles or subsurface white bands at aging times longer than 4 years post gamma irradiation. In contrast, devices made by directly molded 1900 resin showed no significantly increased oxidation (<.945g/cc) levels at aging times up to 9 years post irradiation. This same oxidation resistance phenomena was seen in the retrieved components made from directly molded 1900 resins. There were nonconsolidated particles found in several of the 1900 components and bands found in 1 component. There was no correlation of particles or bands with implant or aging times. Further, there were no signs of fracture or de|aminstion in 10 of 11 of these retrieved molded components. Discussion: There is a surprising and significant effect of resin and processing method on the oxidation behavior of UHMWPE components. In strong contrast to machined UHMWPE devices, components made from directly molded 1900 resins do not appear to oxidize significantly after gamma irradiation after 9 years post irradiation aging or clinical use. Retrieved devices made from directly molding 1900 resins, were found to have a very low incidence of delamination and
* Denotes that something Presenters are boldfaced.
of vaIue was received.
Charles J. Pearlman, MD, New York, NY, Jess Lonner, MD, FredexickJ. Kumar, PhD, William L. Jaffe, MD Purpose: To evaluate the effect of different keel designs on tibial tray micromotion in both osteoporotic and nonosteoporotic tibial models. Summary of Methods and Results: Four cemented tihial trays with differing peg and keel designs were tested. Sawbones tibiae with approximately the same elastic properties and shape as cortical human bone, were used. A two-part polyurethane foam was prepared in two consistencies, simulating osteoporotlc and nonosteoporotic bone, and was used to simulate the proximal tihial cancelous bone. Tibial trays were implanted into the models with bone cement only beneath the trays themselves, leaving the keels and pegs uncemented. Specimens were loaded with eccentric medial forces using a MTS servohydraulic testing machine at full extension, at 30 ° at 45 ° of flexion. Each tihial component was tested three times for each of the two differing trabecular bone densities. Tray liftoff and toggle were measured using an array of six linear variable displacement transducers. Results of both non-osteoporotic and osteoporotie models revealed only minor differences in stability between keel designs. When all models were tested at full extension and progressive angles of flexion, they demonstrated less than 25 microns of motion in anterior, lateral and posterior planes, (p. 05). Conclusion: When fixed with bone cement, none of the keel designs tested showed superior ability in either non-osteoporotic or osteoporotic models to resist micromotion when subjected to eccentric loads. Prosthetic keel design may not play an important role in the development of tibial tray loosening in osteoporotic patients whose components are fixed with bone cement.
POSTER# 6 THE EFFECT OF OSTEOPOROSIS ON THE CANCELLOUS BONE DISTRIBUTION OF THE PROXIMAL TIBIA Philip C. Noble, PhD, Houston, TX, Allan Elliott, M. Darryl Antonancci, MD, H.S. TuUos, MD Purpose: To measure the distribution of cancellous bone density within the proximal tibia and to document changes secondary to osteoporosis. Conclusion: The distribution of cancellous bone within the proximal tibia is highly non-uniform and is concentrated posteriorly, antero-mediaIly and beneath the weight-bearing surfaces. This distribution changes with osteoporosis. Significance: For stable fixation of tihial prostheses, purchase must be obtained in bone of adequate strength beyond the central and anterior thirds of the metaphysis. This becomes especially critical when cementing all-poly components into nsteoporotic tibiae. S u m m a r y of Method, Results, and Discussion: Ten tibiae were obtained from cadaveric donors ranging from 43 to 93 years of age (5 males, 5 females). Six specimens were graded as normal, four as osteoporofic. Transverse slices were cut from each specimen from the medial plateau to the tibial tubercule. The distribution of trabecular density in each slice was measured using high resolution videodensitometry of contact radiographs. The average trabecular density of the ten metaphyseal specimens ranged from 0.320 to 0.651 gm/cm3. The medial plateau had an average density of 0.59 ± 0.06 grrdemB, 18% higher than the lateral plateau (0.50 ± 0.0~c gm/cm3). The weakest bone was found within the central third (0.36 ± 0.03 gm/cm3) and within the anterior third (0.42 ± 0.03 gm/cm3) of the tihial metaphysis. In the osteoporotic specimens, the medial and lateral plateau were of identical bone density (0.39 ± 0.02 gm/cm3) with relatively stronger bone antero-mcdially.
The Journal of Arthroplasty Vol. 12 No. 2 February 1997
Discussion & Conclusion: The standard medial parapatellar approach devascularizes the medial branches leading to the patella. If the dissection stays within the retinacuhim and not in the muscle, it does not disturb the neural innervation to the VMC. The subvastus approach maintains the medial blood supply to the patella. If the saphenons nerve branch to the VMC exists, this approach does destroy this distal innervation to the VMC. If the subvastns approach dissects proximally about 12 cm proximal to the joint line, traction injury to the deep branch to the VMC could occur. The midvastns approach does not endanger the superficial, deep, or saphenons nerve innervation to the VMC or the vasculariza-. tion of the patella. If the dissection proceeds for more than 6 cm from the superomedial pole of the patella, the approach could interfere with the deep VMC branch. The advantages and disadvamages of all three approaches should be contemplated before TKA dissection is performed.
fracture despite the large contact stresses associated with nonconforming designs. This result indicates the complexity of oxidation processes that occur in vivo and that methods to minimize oxidation by changing sterilization methods should be evaluated closely. It is interesting to note that the lnsall/Burstein I, a conforming knee design, was also made from directly molding of 1900 resin and has a 93% survivorship at 13 years of implantation. If oxidation is a factor in the performance of total joints, directly molding with 1900 resin may provide long term performance advantages.
POSTER# 5 THE EFFECTS OF KEEL DESIGN ON TIBIAL COMPONENT STABILITY
POSTER# 4 EFFECTS OF RESIN TYPE & MANUFACTURING PROCESS ON THE OXIDATION OF UHMWPE COMPONENTS Stephen Li, PhD, New York, NY, B.D. Furman, BS, M.A. Ritter, MD Introduction: The contact stresses for total knee components of different designs have been reported to vary from 20 to 45 MPA. However, performance of these components, as measured by polyethylene damage, wear and length of implantation are not a strict function of these stresses or subsurface shear stresses. A possible explanation of this inability to predict specific performance from modeling results may lie in variations in the polyethylene quality or material properties. Further, quality and material properties may also be greatly altered by oxidation processes. Oxidation of gamma sterilized UHMWPE used in joint arthroplasty has been shown to continue for over 10 years after irradiation. This oxidative aging can cause severe embrittlement and the formation of subsurface defects and 'white bands' within the polyethylene. However, these reports did not consider the potential sensitivity of the oxidation process to different UHMWPE resin types or differences in manufacturing method. We report the analysis and comparison of oxidation and UHMWPE quality of 15 never implanted devices made from 4 different polyethylene types and 2 different manufacturing processes at 2 to 13 years post gamma irradiation. These results will be compared to the analysis of retrieved molded tihiaI components after implant times of 1 to 11 years. M a t e r i a l s a n d M e t h o d s : Never implanted c o m p o n e n t s were made from m a c h i n e d 4 1 5 G U R ( 2 ) , R C H 1 0 0 0 ( 3 ) , H y l a m e r ( 2 ) and directly m o l d e d 1900cm(8). All were gamma sterilized 2-13 years prior to examination. The components made with 1900 resin were all directly molded and all other resins were machined into the final device shape. The eleven retrieved tibial inserts were of two different, nonconforming, knee designs made from directly molded 1900 resin and were retrieved after 1-11 years implantation. Oxidation was assessed by density, FTIR and crystallinity measurements using previously reported methods. Polyethylene quality was determined by examination of cross sections of the components. Results: Never implanted components made from 415GUR, RCH1000 and Hylamer all showed very high oxidation levels (>.96g/cc) or exhibited high levels of nonconsolidated particles or subsurface white bands at aging times longer than 4 years post gamma irradiation. In contrast, devices made by directly molded 1900 resin showed no significantly increased oxidation (<.945g/cc) levels at aging times up to 9 years post irradiation. This same oxidation resistance phenomena was seen in the retrieved components made from directly molded 1900 resins. There were nonconsolidated particles found in several of the 1900 components and bands found in 1 component. There was no correlation of particles or bands with implant or aging times. Further, there were no signs of fracture or de|aminstion in 10 of 11 of these retrieved molded components. Discussion: There is a surprising and significant effect of resin and processing method on the oxidation behavior of UHMWPE components. In strong contrast to machined UHMWPE devices, components made from directly molded 1900 resins do not appear to oxidize significantly after gamma irradiation after 9 years post irradiation aging or clinical use. Retrieved devices made from directly molding 1900 resins, were found to have a very low incidence of delamination and
* Denotes that something Presenters are boldfaced.
of vaIue was received.
Charles J. Pearlman, MD, New York, NY, Jess Lonner, MD, FredexickJ. Kumar, PhD, William L. Jaffe, MD Purpose: To evaluate the effect of different keel designs on tibial tray micromotion in both osteoporotic and nonosteoporotic tibial models. Summary of Methods and Results: Four cemented tihial trays with differing peg and keel designs were tested. Sawbones tibiae with approximately the same elastic properties and shape as cortical human bone, were used. A two-part polyurethane foam was prepared in two consistencies, simulating osteoporotlc and nonosteoporotic bone, and was used to simulate the proximal tihial cancelous bone. Tibial trays were implanted into the models with bone cement only beneath the trays themselves, leaving the keels and pegs uncemented. Specimens were loaded with eccentric medial forces using a MTS servohydraulic testing machine at full extension, at 30 ° at 45 ° of flexion. Each tihial component was tested three times for each of the two differing trabecular bone densities. Tray liftoff and toggle were measured using an array of six linear variable displacement transducers. Results of both non-osteoporotic and osteoporotie models revealed only minor differences in stability between keel designs. When all models were tested at full extension and progressive angles of flexion, they demonstrated less than 25 microns of motion in anterior, lateral and posterior planes, (p. 05). Conclusion: When fixed with bone cement, none of the keel designs tested showed superior ability in either non-osteoporotic or osteoporotic models to resist micromotion when subjected to eccentric loads. Prosthetic keel design may not play an important role in the development of tibial tray loosening in osteoporotic patients whose components are fixed with bone cement.
POSTER# 6 THE EFFECT OF OSTEOPOROSIS ON THE CANCELLOUS BONE DISTRIBUTION OF THE PROXIMAL TIBIA Philip C. Noble, PhD, Houston, TX, Allan Elliott, M. Darryl Antonancci, MD, H.S. TuUos, MD Purpose: To measure the distribution of cancellous bone density within the proximal tibia and to document changes secondary to osteoporosis. Conclusion: The distribution of cancellous bone within the proximal tibia is highly non-uniform and is concentrated posteriorly, antero-mediaIly and beneath the weight-bearing surfaces. This distribution changes with osteoporosis. Significance: For stable fixation of tihial prostheses, purchase must be obtained in bone of adequate strength beyond the central and anterior thirds of the metaphysis. This becomes especially critical when cementing all-poly components into nsteoporotic tibiae. S u m m a r y of Method, Results, and Discussion: Ten tibiae were obtained from cadaveric donors ranging from 43 to 93 years of age (5 males, 5 females). Six specimens were graded as normal, four as osteoporofic. Transverse slices were cut from each specimen from the medial plateau to the tibial tubercule. The distribution of trabecular density in each slice was measured using high resolution videodensitometry of contact radiographs. The average trabecular density of the ten metaphyseal specimens ranged from 0.320 to 0.651 gm/cm3. The medial plateau had an average density of 0.59 ± 0.06 grrdemB, 18% higher than the lateral plateau (0.50 ± 0.0~c gm/cm3). The weakest bone was found within the central third (0.36 ± 0.03 gm/cm3) and within the anterior third (0.42 ± 0.03 gm/cm3) of the tihial metaphysis. In the osteoporotic specimens, the medial and lateral plateau were of identical bone density (0.39 ± 0.02 gm/cm3) with relatively stronger bone antero-mcdially.