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Department of Mechanical Systems Engineering
Biomechanics, Orthpaedic Surgery, Medical Engineering, Ergonomics
Yoshimori Kiriyama, Yudai Sato, Yota Muramatsu, Teppei Mano, Katsumasa Tanaka, and Kotaro Oshio
BMC Musculoskeletal Disorders, eISSN: 14712474, Published: December 2022 Springer Science and Business Media LLC
Abstract Background A hairline crack on the cranium can occur even under a small external load or impact and are thus often observed in patients who have experienced an accidental fall or collision. Typical finite element analysis is useful to analyze the stress concentration or the propagation of stress waves. However, a stress propagation model does not accurately reproduce the features of hairline cracks on the cranium. The objective in this study was to reproduce cranial hairline cracks. Methods A three-dimensional finite element model of the cranial bone was developed from a patient CT images. The model consists of the frontal, parietal, occipital and temporal bones, and the bones are connected with the sutures. Additionally, the model comprised three layers; the external and internal tables and the diploe. The model was analyzed using the extended finite element method (X-FEM), and a forming limit diagram (FLD) was embedded in the model. In this study, the model was symmetrized bilaterally using the model developed from the left side of the skull. The FLD in this study was assumed to be a relationship between the maximum and minimum strains when a fracture occurs. A total of 13 typical loadings were applied to the model: loading points on the top, left, and back of the cranium were considered, and at each loading point, loads were applied with four or five different directions, namely perpendicular to the cranium and inclined in the anterior, posterior, superior, or inferior at an angle of 45∘. Results Under all loading conditions, many small cracks formed radially at the loading points. Moreover, some large cracks formed under the certain loading conditions. The crack shapes on the top and left side could be associated with the specific loading directions, whereas cracks on the back did not show distinguishing characteristics depending on the loading directions. The present model was reproduced anatomically and morphologically, and the results were similar to those obtained in previous cadaver experiments. Conclusions Through X-FEM analysis of the FE model embedded with an FLD, hairline cracks in the cranium were reproduced, and a few crack shapes were identified as potential markers for estimating the loading conditions.
Yuki Kitta, Yoshimori Kiriyama, Kengo Harato, Shu Kobayashi, Yasuo Niki, Morio Matsumoto, Masaya Nakamura, and Takeo Nagura
Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, ISSN: 09544119, eISSN: 20413033, Volume: 236, Pages: 566-572, Published: April 2022 SAGE Publications
Direct measurement of cartilage stiffness provides useful clinical information and enables us to develop treatment strategies for patients. We applied an indentation sensor to evaluate cartilage stiffness under arthroscopic control. The purpose of this study was to validate the arthroscopic indentation sensor using cadaver knees and to measure cartilage stiffness in clinical cases. The stiffness of a material with known properties was measured at thicknesses from 2 mm to 10 mm with a 2-mm interval. This was repeated three times at each thickness to evaluate repeatability. The articular cartilage stiffness of the medial and lateral femoral condyles of five human cadaveric knees was measured. The sensor was inclined from 0° to 20° with 1° intervals. The stiffness value at each degree of inclination was compared to evaluate the acceptable measuring angle. Additionally, articular cartilage stiffness was measured in 23 adolescent and 11 adult patients under arthroscopy. Young’s moduli of the material were 1.15–1.24 (mean 1.20) MPa. Inter-class correlation coefficients in repeated measurements using the material were 0.83–0.99. There were no differences in the cartilage stiffness between the medial and lateral femoral condyles of the cadaver knees. All condyles showed a nonlinear relationship between force and displacement. The force decreased in all condyles when the tip of the sensor system was tilted. The range of error was < 97.1% within 5° inclination. There was a moderate negative correlation between age and cartilage stiffness in adolescent patients, and a moderate positive correlation in adult patients. Since the sensor system is manually held during measurement, the validity and repeatability to assess material properties of the cartilage may be inaccurate. This study has proven that the instrument can measure the stiffness of joint cartilage reliably and is a useful clinical tool under arthroscopic control.
Naoko TAMADA and Yoshimori KIRIYAMA
Journal of Biomechanical Science and Engineering, ISSN: 18809863, Published: 2022 Japan Society of Mechanical Engineers
Teppei MANO, Shigehiro HASHIMOTO, Masafumi MACHIDA, and Yoshimori KIRIYAMA
Journal of Biomechanical Science and Engineering, ISSN: 18809863, Pages: 1-13, Published: 2021 Japan Society of Mechanical Engineers
Satoshi Oki, Noboru Matsumura, Yoshimori Kiriyama, Takuji Iwamoto, Kazuki Sato, and Takeo Nagura
Journal of Orthopaedic Trauma, ISSN: 08905339, eISSN: 15312291, Pages: e385-e389, Published: 1 November 2017 Ovid Technologies (Wolters Kluwer Health)
OBJECTIVE The purpose of this study was to describe the 3-dimensional deformities of midshaft clavicle fractures, which had been treated nonoperatively, using computed tomography (CT) surface matching. METHODS Twenty-one patients with unilateral midshaft clavicle fracture, who had been treated nonoperatively, were enrolled and evaluated retrospectively. The 3-dimensional deformity of the fractured clavicle was evaluated by CT surface matching. CT scans of 21 age- and sex-matched patients with initial traumatic shoulder dislocation or proximal humeral fracture were enrolled as a control group, and the differences in 3-dimensional deformities and lengths of the clavicles between the fracture group and the control group were evaluated. A correlation analysis was also performed between rotational deformities and clavicular length shortening. RESULTS The affected clavicle showed 1.3 ± 6.9 degrees of downward angular deformity, 2.1 ± 8.0 degrees of anterior angular deformity, and 5.0 ± 4.9 degrees of anterior rotational deformity. Compared with the control group, the fractured clavicle showed larger anterior rotational deformity (P = 0.021). Shortening of the clavicle demonstrated negative correlation with anterior axial rotation (R = -0.534, P = 0.013), but no correlation was found between clavicular shortening and the other 2 rotational deformities. CONCLUSION In cases of midshaft clavicle fracture, the distal fragment usually rotates anteriorly because of its anatomical relationships. Shortening deformity after clavicle fracture was reported to change shoulder kinematics, and anterior rotational deformity might adversely affect scapular motion.
Takeo Nagura, Yasuo Niki, Kengo Harato, Takeshi Mochizuki, and Yoshimori Kiriyama ISSN: 09680160, eISSN: 18735800, Pages: 250-257, Published: 1 March 2017 Elsevier BV
BACKGROUND Analysis of dynamic knee loading during gait is essential to prevent mechanical failures following total knee arthroplasty. External knee adduction moment during gait is the primary factor producing medial joint reaction force, and an increase in the moment is directly related to an increase in the medial compartment load on the knee. METHODS Knee adduction moment during gait in 39 knees of 32 female patients following a posterior stabilized knee replacement with a single surgeon was evaluated at 1.3months following surgery. A cut-off moment was determined as mean+1 standard deviation (SD) of the moment from 10 healthy subjects, and patients' knees were divided into high- and normal-moment groups. Significant differences in clinical assessments and gait parameters between the two groups were assessed. RESULTS Based on the cut-off moment, 23 knees were grouped into normal knees and 16 knees were grouped into high-moment knees. High-moment knees showed identical femorotibial angles and knee society scores but had greater toe-out angles and medially directed ground reaction forces compared to normal-moment knees. High-moment knees showed strong correlations between peak moment and knee adduction angle, and frontal plain moment arm. CONCLUSIONS The clinical significance of a high knee adduction moment following total knee arthroplasty remains unclear, but dynamic frontal alignment during gait is one of the key factors for residual high-moment knees following surgery.
Kentaro Takeda, Takayuki Hasegawa, Yoshimori Kiriyama, Hideo Matsumoto, Toshiro Otani, Yoshiaki Toyama, and Takeo Nagura
Journal of Biomechanics, ISSN: 00219290, eISSN: 18732380, Pages: 2526-2530, Published: 18 July 2014 Elsevier BV
The purpose of this study was to determine whether mechanical adaptations were present in patients with anterior cruciate ligament (ACL)-deficient knees during high-demand activities. Twenty-two subjects with unilateral ACL deficiency (11 males and 11 females, 19.6 months after injury) performed five different activities at a comfortable speed (level walking, ascending and descending steps, jogging, jogging to a 90-degree side cutting toward the opposite direction of the tested side). Three-dimensional knee kinematics for the ACL-deficient knees and uninjured contralateral knees were evaluated using the Point Cluster Technique. There was no significant difference in knee flexion angle, but an offset toward the knee in less valgus and more external tibial rotation was observed in the ACL-deficient knee. The tendency was more obvious in high demand motions, and a significant difference was clearly observed in the side cutting motions. These motion patterns, with the knee in less valgus and more external tibial rotation, are proposed to be an adaptive movement to avoid pivot shift dynamically, and reveal evidence in support of a dynamic adaptive motion occurring in ACL-deficient knees.
Yoshimori Kiriyama, Hideo Matsumoto, Yoshiaki Toyama, and Takeo Nagura
Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, ISSN: 09544119, eISSN: 20413033, Volume: 228, Pages: 140-148, Published: February 2014 SAGE Publications
The aim of this study was to develop a new suture tension sensor for musculoskeletal soft tissue that shows deformation or movements. The suture tension sensor was 10 mm in size, which was small enough to avoid conflicting with the adjacent sensor. Furthermore, the sensor had good linearity up to a tension of 50 N, which is equivalent to the breaking strength of a size 1 absorbable suture defined by the United States Pharmacopeia. The design and mechanism were analyzed using a finite element model prior to developing the actual sensor. Based on the analysis, adequate material was selected, and the output linearity was confirmed and compared with the simulated result. To evaluate practical application, the incision of the skin and capsule were sutured during simulated total knee arthroplasty. When conventional surgery and minimally invasive surgery were performed, suture tensions were compared. In minimally invasive surgery, the distal portion of the knee was dissected, and the proximal portion of the knee was dissected additionally in conventional surgery. In the skin suturing, the maximum tension was 4.4 N, and this tension was independent of the sensor location. In contrast, the sensor suturing the capsule in the distal portion had a tension of 4.4 N in minimally invasive surgery, while the proximal sensor had a tension of 44 N in conventional surgery. The suture tensions increased nonlinearly and were dependent on the knee flexion angle. Furthermore, the tension changes showed hysteresis. This miniature tension sensor may help establish the optimal suturing method with adequate tension to ensure wound healing and early recovery.
Yoshimori Kiriyama, Kota Watanabe, Morio Matsumoto, Yoshiaki Toyama, and Takeo Nagura
Journal of Biomechanics, ISSN: 00219290, eISSN: 18732380, Pages: 302-307, Published: 3 January 2014 Elsevier BV
The objective of this study was to quantify the three-dimensional spatial strain distribution of a scoliotic spine by nonhomogeneous transformation without using a statistically averaged reference spine. The shape of the scoliotic spine was determined from computed tomography images from a female patient with adolescent idiopathic scoliosis. The shape of the scoliotic spine was enclosed in a rectangular grid, and symmetrized using a thin-plate spline method according to the node positions of the grid. The node positions of the grid were determined by numerical optimization to satisfy symmetry. The obtained symmetric spinal shape was enclosed within a new rectangular grid and distorted back to the original scoliotic shape using a thin-plate spline method. The distorted grid was compared to the rectangular grid that surrounded the symmetrical spine. Cobb's angle was reduced from 35° in the scoliotic spine to 7° in the symmetrized spine, and the scoliotic shape was almost fully symmetrized. The scoliotic spine showed a complex Green-Lagrange strain distribution in three dimensions. The vertical and transverse compressive/tensile strains in the frontal plane were consistent with the major scoliotic deformation. The compressive, tensile and shear strains on the convex side of the apical vertebra were opposite to those on the concave side. These results indicate that the proposed method can be used to quantify the three-dimensional spatial strain distribution of a scoliotic spine, and may be useful in quantifying the deformity of scoliosis.
Hidenori Tanikawa, Hideo Matsumoto, Kengo Harato, Yoshimori Kiriyama, Yasunori Suda, Yoshiaki Toyama, and Takeo Nagura
Journal of Orthopaedic Science, ISSN: 09492658, eISSN: 14362023, Pages: 695-696, Published: July 2014 Elsevier BV
Hidenori Tanikawa, Kengo Harato, Yasunori Suda, Yoshiaki Toyama, Hideo Matsumoto, Yoshimori Kiriyama, and Takeo Nagura
Journal of Orthopaedic Science, ISSN: 09492658, eISSN: 14362023, Pages: 104-111, Published: January 2014 Elsevier BV
BackgroundA variety of athletic exercises are performed in sports training or rehabilitation after knee injuries. However, it remains unclear whether males and females exhibit similar joint loading during the various athletic motions. The purpose of this study was to identify gender differences in knee biomechanics during the athletic motions.MethodsThree-dimensional knee kinematics and kinetics were investigated in 20 recreational athletes (10 males and 10 females) while jumping rope, backward running, side running, side-to-side running, side-to-forward running, inside turning, and outside turning. The strengths of the quadriceps and hamstring muscles, the knee joint force, the knee joint angle, and the knee joint moment were compared between males and females using one-tailed t tests.ResultsPeak knee anterior force was greater in female recreational athletes than in their male counterparts during jumping rope, side-to-forward running, inside turning, and outside turning. Female subjects displayed greater peak knee abduction angles and greater peak knee flexion moments while jumping rope compared to their male counterparts. There were no significant differences between the sexes in knee kinematics and kinetics in the frontal and transverse planes during running and turning motions.ConclusionsFemale recreational athletes exhibited significantly different knee biomechanics compared with male counterparts during jumping rope and turning motions.
Satoshi Oki, Noboru Matsumura, Wataru Iwamoto, Hiroyasu Ikegami, Yoshimori Kiriyama, Toshiyasu Nakamura, Yoshiaki Toyama, and Takeo Nagura
Journal of Shoulder and Elbow Surgery, ISSN: 10582746, eISSN: 15326500, Pages: 1433-1439, Published: October 2013 Elsevier BV
HYPOTHESIS We hypothesized that the clavicle overrides the acromion during certain shoulder motions for individuals with acromioclavicular (AC) joint separation producing clinical symptoms. We measured 3-dimensional clavicular and scapular motions in AC joint separation models during humerothoracic motions, which should be impacted by the loss of AC joint continuity. MATERIALS AND METHODS Ten shoulders from 6 whole cadavers were used. The scapular and clavicular motions were measured in intact and AC joint separation models using an electromagnetic tracking device. The measurement was performed during shoulder abduction with humerothoracic neutral rotation. It was also measured during shoulder abduction with humerothoracic internal rotation, which could cause clavicular overriding. The kinematic changes caused by ligament sectioning were evaluated in these 2 arm motions. RESULTS The clavicle completely overrode the acromion in all AC separation models during abduction with internal rotation, but not in any shoulders during abduction with neutral rotation. Upward clavicular rotation increased, posterior clavicular rotation decreased, and external scapular rotation decreased with ligament sectioning. These kinematic changes were common for both of the measured arm motions. Scapular upward rotation and posterior tilt did not change because of ligament sectioning during abduction with neutral rotation. However, these scapular rotations significantly decreased with ligament sectioning during shoulder abduction with internal rotation. CONCLUSION Scapular and clavicular kinematics were affected in AC separation models. Abduction with humeral internal rotation resulted in a decrease in scapular posterior tilt and upward rotation in AC separation models, and thereby could lead to AC joint articulation dysfunction.
Hidenori Tanikawa, Hideo Matsumoto, Ikki Komiyama, Yoshimori Kiriyama, Yoshiaki Toyama, and Takeo Nagura
Journal of Applied Biomechanics, ISSN: 10658483, eISSN: 15432688, Pages: 749-755, Published: December 2013 Human Kinetics
It has been suggested that noncontact anterior cruciate ligament injury commonly occurs during sports requiring acute deceleration or landing motion and that female athletes are more likely to sustain the injury than male athletes. The purpose of this study was to make task-to-task and male-female comparisons of knee kinematics and kinetics in several athletic activities. Three-dimensional knee kinematics and kinetics were investigated in 20 recreational athletes (10 males, 10 females) while performing hopping, cutting, turning, and sidestep and running (sharp deceleration associated with a change of direction). Knee kinematics and kinetics were compared among the four athletic tasks and between sexes. Subjects exhibited significantly lower peak flexion angle and higher peak extension moment in hopping compared with other activities (P< .05). In the frontal plane, peak abduction angle and peak adduction moment in cutting, turning, and sidestep and running were significantly greater compared with hopping (P< .05). No differences in knee kinematics and kinetics were apparent between male and female subjects. Recreational athletes exhibited different knee kinematics and kinetics in the four athletic motions, particularly in the sagittal and frontal planes. Male and female subjects demonstrated similar knee motions during the four athletic activities.
Satoshi Oki, Noboru Matsumura, Wataru Iwamoto, Hiroyasu Ikegami, Yoshimori Kiriyama, Toshiyasu Nakamura, Yoshiaki Toyama, and Takeo Nagura
American Journal of Sports Medicine, ISSN: 03635465, eISSN: 15523365, Pages: 2617-2626, Published: November 2012 SAGE Publications
Background: Scapulothoracic dyskinesis is an important consequence of acromioclavicular joint dislocations. However, no reports have described changes in 3-dimensional motions of the scapula and clavicle with respect to the thorax caused by acromioclavicular joint dislocation. Hypothesis: Sectioning of the acromioclavicular (AC) and coracoclavicular (CC) ligaments affects scapular and clavicular motion in a whole-cadaver model. Study Design: Controlled laboratory study. Methods: We evaluated shoulder girdle motion (scapula, clavicle, and humerus) relative to the thorax of 14 shoulders from 8 whole cadavers after sequential sectioning of the AC and CC ligaments (trapezoid and conoid ligaments). An electromagnetic tracking device measured 3-dimensional kinematics of the scapula and clavicle during humerothoracic elevation in the coronal and sagittal planes and adduction in the horizontal plane. Results: Sectioning of the AC ligament increased clavicular retraction during sagittal plane elevation and horizontal plane adduction. Sectioning of the trapezoid ligament decreased scapular external rotation during sagittal plane elevation and horizontal plane adduction. Sectioning of the conoid ligament decreased scapular posterior tilting during sagittal plane elevation and horizontal plane adduction. Acromioclavicular and CC ligament sectioning also delayed clavicular posterior rotation and increased clavicular upward rotation during coronal plane elevation. Conclusion: Our study revealed that AC and CC ligament disruption affected in vitro shoulder girdle kinematics in the whole-cadaver model. Clinical Relevance: The results of this cadaveric study revealed that AC and CC ligament disruption could cause dyskinesis of the scapula and clavicle. The kinematic changes could be a potential source of pain and dysfunction in the shoulder with AC joint dislocation, and therefore surgical reconstruction may be indicated in certain patients.
Yuji Kuroyanagi, Takeo Nagura, Yoshimori Kiriyama, Hideo Matsumoto, Toshiro Otani, Yoshiaki Toyama, and Yasunori Suda ISSN: 09680160, eISSN: 18735800, Pages: 130-134, Published: March 2012 Elsevier BV
Varus thrust is an abnormal lateral knee motion frequently seen in patients with medial knee osteoarthritis (OA) during gait. It is a worsening of the alignment in the stance phase of the gait cycle and closely relates to disease progression. In this study, we measured the thrust quantitatively using skin markers and examined the relationship to other static and dynamic parameters. Forty-four knees in 32 patients (mean age, 72years; range, 64-81years) who exhibited the radiographic OA at least grade 2 according to the Kellgren-Lawrence (K-L) scale were enrolled. Gait analysis was performed for each patient to measure the amount of thrust and knee adduction moment. The amounts of thrust in subjects with K-L grades 2 (25 knees), 3 (13 knees), and 4 (6 knees) were 2.4°(±1.3°), 2.8°(±1.4°), and 7.2°(±5.3°), respectively and the knee adduction moments were 3.6(±1.5) %BW⁎Ht, 3.9(±1.2) %BW⁎Ht and 6.9(±2.2%) BW⁎Ht, respectively. The amount of thrust also exhibited significant correlation to static radiographic alignment (R=0.47: 95% confidence interval 0.67-0.21, p=0.0038) and showed greater correlation to the knee adduction moment (R=0.73: 95% confidence interval 0.84-0.55, p<0.001), which has been identified as an important dynamic index of the disease. The amount of thrust, which is able to be measured by simple inexpensive equipment, correlated to static and dynamic parameters and may offer an important clinical index for knee OA.
Kensuke Sakai, Yoshimori Kiriyama, Hiroo Kimura, Noriaki Nakamichi, Toshiyasu Nakamura, Hiroyasu Ikegami, Hideo Matsumoto, Yoshiaki Toyama, and Takeo Nagura
Journal of Shoulder and Elbow Surgery, ISSN: 10582746, Pages: 86-90, Published: January 2010 Elsevier BV
HYPOTHESIS Throwing fractures of the humerus are well known, but the exact mechanism of injury is not clear. It has been postulated that these may be stress fractures because the forces have not seemed sufficient to cause acute fractures while throwing. MATERIALS AND METHODS Using finite element analysis, we reproduced fractures of the humerus using 3-dimensional models built from computed tomography images of 5 healthy volunteers. To apply the load during throwing, we assumed that the humeral head was completely fixed, and external rotation torque was applied to the distal end of the humerus until the stress of the bone elements became greater than yield stress. We reproduced the fracture line by removing the bone elements. RESULTS The maximum stress concentration was seen in the distal shaft, where a typical spiral fracture was created in all cases. In the humeral models, the torque required to initiate fracture ranged from 51 to 70 Nm. A strong correlation existed between the torque required to initiate fracture and thickness of the humeral cortical bone (R(2) = 0.74). CONCLUSION These results indicate that thickness of the humerus represents one factor contributing to fractures that occur while throwing. LEVEL OF EVIDENCE Basic science study.
Yoshimori Kiriyama, Morio Matsumoto, Yoshiaki Toyama, Thomas P. Andriacchi, and Takeo Nagura
Proceedings of the ASME Summer Bioengineering Conference 2009, SBC2009, Issue: PART B, Pages: 1241-1242, Published: 2009 American Society of Mechanical Engineers
A spinal instrument with pedicle screws is one of the most effective surgical techniques for correction of spinal deformity. However, when excessive mechanical loads are applied on the implant upon the fixation, there is a concern for back out of the screw in the cases with poor bone condition such as osteoporosis. For that reason, the mechanical loads at the bone-screw interface should be analyzed for safe and successful correction.
Yoshimori KIRIYAMA, Nobutoshi YAMAZAKI, Takeo NAGURA, Morio MATSUMOTO, Kazuhiro CHIBA, and Yoshiaki TOYAMA
JSME International Journal, Series C: Mechanical Systems, Machine Elements and Manufacturing, ISSN: 13447653, eISSN: 1347538X, Pages: 577-585, Published: 15 June 2006 Japan Society of Mechanical Engineers
In segmental pedicle screw instrumentation, the relationship between the combinations of pedicle screw placements and the degree of deformity correction was investigated with a three-dimensional rigid body and spring model. The virtual thoracolumbar scoliosis (Cobb’s angle of 47 deg.) was corrected using six different combinations of pedicle-screw placements. As a result, better correction in the axial rotation was obtained with the pedicle screws placed at or close to the apical vertebra than with the screws placed close to the end vertebrae, while the correction in the frontal plane was better with the screws close to the end vertebrae than with those close to the apical vertebra. Additionally, two screws placed in the convex side above and below the apical vertebra provided better correction than two screws placed in the concave side. Effective deformity corrections of scoliosis were obtained with the proper combinations of pedicle screw placements.
Takeo Nagura, Hideo Matsumoto, Yoshimori Kiriyama, Ajit Chaudhari, and Thomas P. Andriacchi
Journal of Applied Biomechanics, ISSN: 10658483, eISSN: 15432688, Pages: 305-313, Published: November 2006 Human Kinetics
The aim of the study was to estimate the tibiofemoral joint force in deep flexion to consider how the mechanical load affects the knee. We hypothesize that the joint force should not become sufficiently large to damage the joint under normal contact area, but should become deleterious to the joint under the limited contact area. Sixteen healthy knees were analyzed using a motion capture system, a force plate, a surface electromyography, and a knee model, and then tibiofemoral joint contact forces were calculated. Also, a contact stress simulation using the contact areas from the literature was performed. The peak joint contact forces (M±SD) were 4566 ± 1932 N at 140 degrees in rising from full squat and 4479 ± 1478 N at 90 degrees in rising from kneeling. Under normal contact area, the tibiofemoral contact stresses in deep flexion were less than 5 MPa and did not exceed the stress to damage the cartilage. The contact stress simulation suggests that knee prosthesis having the contact area smaller than 200 mm2may be problematic since the contact stress in deep flexion would become larger than 21 MPa, and it would lead damage or wear of the polyethylene.