- Email: [email protected]
P074 Preclinical biomechanical evaluation of a new, proximally cemented hip stem
PRECLINICAL
BIOMECHANICAL EVALUATION OF A NEW, PROXIMALLY CEMENTED HIP STEM M. Angst, Ch. Fankhauser Dr Robert Mathys Foundation, Switzerland
INTRODUCTION: In THA, sufficient initial stability of the implant is considered a prerequisite for long-term survival [1,2]. Early stem fixation and physiological loading in the intertrochanteric region is realized by a design and surgical procedure that allow cement injection into a proximal, collarshaped recess after the stem has been placed in the femur (Option 3000 cemented Mathys Medical Ltd. Switzerland, Fig.1). This new implantation sequence, and the reduced cement bed supporting area raise several questions which require preclinical studies. Inlet ant.
Fig. 1: Channels for cement injection and venting METHODS: a) Qualitative cement-flow studies made on models of the stem’s recess collar b) Low-cycle micromotion measurements by LVDTs on paired human cadaver femurs (comparison to Weber stem, Sulzer Orthopedics, Switzerland) c) Static and dynamic tests in an axial loading mode on human and. bovine cancellous epiphyseal bone blocks (both in dry laboratory environment). Subsidence at the cross-head was recorded at intervals. d) Assuming that the proximal part of the stem cannot resist any varus tilting, the distal portion of the implant would be subjected to high frontal bending, especially in case of isthmus contact. To investigate this aspect, distal-lateral bending tests according to [3] were performed. RESULTS: a) The following surgical technique features could be verified: Injector/riser functionality, merging of the flow fronts, and a self cleaning effect thanks to the riser. b) Micromotion amplitudes in the order of 1 to 30 mm were measured at all sites, and did not differ significantly in the two stems. c) The average static strength of the cement bed was 45kN in the bovine and 6.lkN in the human model (n=4). The subsidence in the bovine model was below 2mm at 3SkN and 5x10’ cycles (6Hz, n=3). d) Distal bending fatigue strength was 2 kN at 6.5~10~ cycles, corresponding to a 7 kN hip-joint load as a conservative estimate. DISCUSSION: a) The cement injection yielded a reproducible, contained and homogenous cement ring. b) The high ML motions present in the fully cemented stem indicate substantial varus tilting. c) Results from static tests showed that the bovine model is adequate to simulate human bone. The ultimate strength can be considered as a measure. to compare different stem designs in terms of their capability to distribute compression and shear loads to the surrounding bone. d) This testing mode can be considered as a worst case. Still, the stem’s frontal bending strength was sufficient. CONCLUSION: The new cementing protocol proved to be feasible and reproducible, and yielded a cement mantle with adequate biomechanical properties in terms of initial stability and postoperative strength of the prosthetic stem. REFERENCES: 1 Freeman, JBJS 76-B:432-81994.2 Mjijberg,‘JBJS 68-B:770-4,1986,3 Heim, SC. exhibit, AAOS, 1995.4 Taylor, JBJS 79-B:181-2, 1997. CORRESPONDENCE: Dr Robert Mathys Foundation; Ch. Fankhauser, Mech Eng; Bischmattstrasse 12; CH-2544 Bettlach. Tel: ++41 32 644 14 04; Fax: ++41 32 644 11 76; [email protected]
92
1 Ith Conference
A RADIOGRAPHIC METHOD TO QUANTIFY IN VIVO KINEMATICS OF THE MOBILE BEARING IN A TKR Ch. Fankhauser’, C. Verboom’, R. Polmans’, S. Himsted? ‘Dr Robert Mathys Foundation, Switzerland, *MH Hannover. Germany INTRODUCTION: Prostheses for Total Knee Arthroplasty (TKA) are available in a wide variety of designs. The design influences both fixation. kinematics, and wear. During recent years mobile polyethylene bearings have been introduced in order to achieve normal knee kinematics. Since movement of the mobile bearings relative to the tibia1 tray is possible a high congruency of the polyethylene surface opposing the femoral component is guaranteed and therefore contact stresses and the risk of increased wear can be reduced. In order to improve a mobile bearing TKA product it is essential to study its kinematics in vivo. For this purpose, a software tool was developed to quantify the relative motion of mobile bearings relative to the tibia1 tray at different knee flexion angles by means of standard radiographs. Both AP translation and internal/external rotation can be quantified. METHODS: The TKR design consists of a polyethylene mobile plateau that is allowed both free rotation about a central peg on the tibiat baseplate, and AP translation (about 4mm).
The femoral component is identical to the fixed inlay design. Markerpoints on standard radiographs are selected on a digitizing tablet (ARISTO Grafik Systeme GmbH & Co. KG, Hamburg) Their spatial positions (A, B, C in Fig.]) are reconstructed by matching the known tray geometry into the raylines (XA’, XB’, XC’). With the known X-ray source distance and the location of the central beam an iterative algorithm calculates the position (rotation/ translation) of the mobile polyethylene plateau relative to the tibial tray.
k2XJLT.S:
The developed program quantifies the in viva kinematics (plateau vs. tibial tray) of a mobile bearing knee design (Fig. 2). The program enables the operator to determine the position of the mobile polyethylene component relative to its tibial baseplate by means of one single x-ray image. Fig.2: Mobile PE plateau DISCUSSION: An interindividual statistic showed that the calculated values are of high accuracy (transl: f 0.3mm; rot: + 0.7) and that interindividual measurements are not &niticantly different for the investigated phantom X-rays (Wilcoxon test: c&.05). A degradation in accuracy is expected by processing clinical material.. CONCLUSION: With this tool changes in the range of motion of the mobile polyethylen bearing (i.e. due to tissue infiltration or tilting of the metal backing) can be detected postoperatively. Together with the patient’s radiological findings this knowledge additionally can confirm the surgeon’s operating technique. REFERENCES: 1 Nilsson K.G. et al. (1997) In vivo kinematics in knee replacements with fixed or mobile polyethylene bearings, 43rd Annual Meeting, ORS 1997, San Francisco. CORRESPONDENCE: Dr Robert Mathys Foundation; Ch. Fankhauser, Mech Eng; Bischmattstrasse 12; CH-2544 Bettlach. Tel: ++4132 644 14 04; Fax: ++4132 644 1176; [email protected]
of the ESB, July 8-11 98, Toulouse.
France
BIOMECHANICAL EVALUATION OF A NEW, PROXIMALLY CEMENTED HIP STEM M. Angst, Ch. Fankhauser Dr Robert Mathys Foundation, Switzerland
INTRODUCTION: In THA, sufficient initial stability of the implant is considered a prerequisite for long-term survival [1,2]. Early stem fixation and physiological loading in the intertrochanteric region is realized by a design and surgical procedure that allow cement injection into a proximal, collarshaped recess after the stem has been placed in the femur (Option 3000 cemented Mathys Medical Ltd. Switzerland, Fig.1). This new implantation sequence, and the reduced cement bed supporting area raise several questions which require preclinical studies. Inlet ant.
Fig. 1: Channels for cement injection and venting METHODS: a) Qualitative cement-flow studies made on models of the stem’s recess collar b) Low-cycle micromotion measurements by LVDTs on paired human cadaver femurs (comparison to Weber stem, Sulzer Orthopedics, Switzerland) c) Static and dynamic tests in an axial loading mode on human and. bovine cancellous epiphyseal bone blocks (both in dry laboratory environment). Subsidence at the cross-head was recorded at intervals. d) Assuming that the proximal part of the stem cannot resist any varus tilting, the distal portion of the implant would be subjected to high frontal bending, especially in case of isthmus contact. To investigate this aspect, distal-lateral bending tests according to [3] were performed. RESULTS: a) The following surgical technique features could be verified: Injector/riser functionality, merging of the flow fronts, and a self cleaning effect thanks to the riser. b) Micromotion amplitudes in the order of 1 to 30 mm were measured at all sites, and did not differ significantly in the two stems. c) The average static strength of the cement bed was 45kN in the bovine and 6.lkN in the human model (n=4). The subsidence in the bovine model was below 2mm at 3SkN and 5x10’ cycles (6Hz, n=3). d) Distal bending fatigue strength was 2 kN at 6.5~10~ cycles, corresponding to a 7 kN hip-joint load as a conservative estimate. DISCUSSION: a) The cement injection yielded a reproducible, contained and homogenous cement ring. b) The high ML motions present in the fully cemented stem indicate substantial varus tilting. c) Results from static tests showed that the bovine model is adequate to simulate human bone. The ultimate strength can be considered as a measure. to compare different stem designs in terms of their capability to distribute compression and shear loads to the surrounding bone. d) This testing mode can be considered as a worst case. Still, the stem’s frontal bending strength was sufficient. CONCLUSION: The new cementing protocol proved to be feasible and reproducible, and yielded a cement mantle with adequate biomechanical properties in terms of initial stability and postoperative strength of the prosthetic stem. REFERENCES: 1 Freeman, JBJS 76-B:432-81994.2 Mjijberg,‘JBJS 68-B:770-4,1986,3 Heim, SC. exhibit, AAOS, 1995.4 Taylor, JBJS 79-B:181-2, 1997. CORRESPONDENCE: Dr Robert Mathys Foundation; Ch. Fankhauser, Mech Eng; Bischmattstrasse 12; CH-2544 Bettlach. Tel: ++41 32 644 14 04; Fax: ++41 32 644 11 76; [email protected]
92
1 Ith Conference
A RADIOGRAPHIC METHOD TO QUANTIFY IN VIVO KINEMATICS OF THE MOBILE BEARING IN A TKR Ch. Fankhauser’, C. Verboom’, R. Polmans’, S. Himsted? ‘Dr Robert Mathys Foundation, Switzerland, *MH Hannover. Germany INTRODUCTION: Prostheses for Total Knee Arthroplasty (TKA) are available in a wide variety of designs. The design influences both fixation. kinematics, and wear. During recent years mobile polyethylene bearings have been introduced in order to achieve normal knee kinematics. Since movement of the mobile bearings relative to the tibia1 tray is possible a high congruency of the polyethylene surface opposing the femoral component is guaranteed and therefore contact stresses and the risk of increased wear can be reduced. In order to improve a mobile bearing TKA product it is essential to study its kinematics in vivo. For this purpose, a software tool was developed to quantify the relative motion of mobile bearings relative to the tibia1 tray at different knee flexion angles by means of standard radiographs. Both AP translation and internal/external rotation can be quantified. METHODS: The TKR design consists of a polyethylene mobile plateau that is allowed both free rotation about a central peg on the tibiat baseplate, and AP translation (about 4mm).
The femoral component is identical to the fixed inlay design. Markerpoints on standard radiographs are selected on a digitizing tablet (ARISTO Grafik Systeme GmbH & Co. KG, Hamburg) Their spatial positions (A, B, C in Fig.]) are reconstructed by matching the known tray geometry into the raylines (XA’, XB’, XC’). With the known X-ray source distance and the location of the central beam an iterative algorithm calculates the position (rotation/ translation) of the mobile polyethylene plateau relative to the tibial tray.
k2XJLT.S:
The developed program quantifies the in viva kinematics (plateau vs. tibial tray) of a mobile bearing knee design (Fig. 2). The program enables the operator to determine the position of the mobile polyethylene component relative to its tibial baseplate by means of one single x-ray image. Fig.2: Mobile PE plateau DISCUSSION: An interindividual statistic showed that the calculated values are of high accuracy (transl: f 0.3mm; rot: + 0.7) and that interindividual measurements are not &niticantly different for the investigated phantom X-rays (Wilcoxon test: c&.05). A degradation in accuracy is expected by processing clinical material.. CONCLUSION: With this tool changes in the range of motion of the mobile polyethylen bearing (i.e. due to tissue infiltration or tilting of the metal backing) can be detected postoperatively. Together with the patient’s radiological findings this knowledge additionally can confirm the surgeon’s operating technique. REFERENCES: 1 Nilsson K.G. et al. (1997) In vivo kinematics in knee replacements with fixed or mobile polyethylene bearings, 43rd Annual Meeting, ORS 1997, San Francisco. CORRESPONDENCE: Dr Robert Mathys Foundation; Ch. Fankhauser, Mech Eng; Bischmattstrasse 12; CH-2544 Bettlach. Tel: ++4132 644 14 04; Fax: ++4132 644 1176; [email protected]
of the ESB, July 8-11 98, Toulouse.
France