Analysis of Mechanical Strength in Talus Using CT-osteoabsorptiometry; in vivo Study

Transcription

1 Analysis of Mechanical Strength in Talus Using CT-osteoabsorptiometry; in vivo Study Yusuke Hara 1), Kazuya Ikoma 1), Masamitsu Kido 1), Kan Imai 1), Masahiro Maki 1), Daisaku Tokunaga 1), Nozomu Inoue 2), Toshikazu Kubo 1) 1) Department of Orthopaedics, Graduate School of Medical Science, Kyoto Prefectural University of Medicine 2) Department of Orthopedic Surgery, Rush University

2 < Analysis of Mechanical Strength in Talus Using CT-osteoabsorptiometry;in vivo Study> < Yusuke Hara, Kazuya Ikoma, Masamitsu Kido, Kan Imai, Masahiro Maki, Daisaku Tokunaga, Nozomu Inoue, Toshikazu Kubo> My disclosure is in the Final AOFAS Program Book. I have no potential conflicts with this presentation.

3 Introduction In clinical practice, the chondral degeneration and progression of osteoarthritis (OA) was diagnosed based on the loss of joint space in radiograph. In the osteoarthritis patients, the joint space width was decreased. The measurement of subchondral bone mineralization is of decisive importance in assessing the functional adaptation of bony tissue to various individual stresses. Müller-Gerbl developed CT-Osteoabsorptiometry (CT-OAM) as a reliable method for assessing the distribution of subchondral mineralization (1). Relationships between strength and subchondral bone density (SBD) of the human joint are known (2). Contact area and intra-articular pressure in the ankle joint have mostly been studied using in vitro cadaveric ankle specimens and finite element models (3). We have reported the in vivo 3D kinematics of tarsal bones in flatfoot using weight-bearing CT scanning. It is important to understand the kinematics of peritalar bones under the weight-bearing accurately in order to reveal and treat foot and ankle disorders. We think that CT-OAM is useful to evaluate the relation between the alteration of the intra-articular width and stress distribution, when we compared the SBD with and without loading. We hypothesized that the intra-articular width especially decreased at high concentrate area of stress distribution. The purpose of this study was to measure the joint space distance and SBD of normal ankle joint surface using a novel 3D measurement method in vivo, and to figure out the relationship between joint space distance and everlasting load stress.

4 Materials and Methods Subjects Eighteen ankles of ten normal subjects between the ages of years (5 males and 5 females) underwent ankle CT scans (IRB approved). The images were taken with using a custom-made loading device of the foot (Rakuhoku Prosthetic and Orthotic Manufacturing Co., Ltd, Kyoto, Japan) that held the hip in 50-degree flexion, the knee in 90- degree flexion, and the foot in neutral position. Setting up the vise between the foot and the knee, the load value was monitored with the scales on the kneepad. Pressure was applied with using the vise with full body-weight. The images of hindfoot bones were reconstructed into 3D model by using CAD software (Mimics, Materialise Inc, Ann Arbor, MI). Joint surface data CT-image data from tibia to talus were converted to point-cloud 3D models. Curved 3D ankle joint surface data was reconstructed. We identified the talus point-cloud data which was within 4.0mm distance as joint surface data (Figure 1). Figure 1: Joint surface data a: sagittal image b: coronal image Figure 1a Figure 1b

5 Measurement of HU Values SBD at each point of talus was calculated in Hounsfield Units (HU). Curved 3D ankle joint surface data were set on the talus joint surface and virtual layers parallel to the talus joint surface were analyzed in 0.5mm increments down to 3.0 mm depth (Figure 2). The mean HU in the talus at each depth was calculated. Surface(0mm depth) 1.5mm depth 3.0mm depth Max Min Figure 2: Measurements of HU

6 Measurement of joint space distance The joint space distance (JSD) was calculated which was shortest distance between talus and tibia joint surface(δtibia-maxhu). In addition, the distance between tibia and the depth of highest HU value of talus was calculated (Figure 3). 4.0mm 4.0mm Figure 3a 0mm Figure 3b 0mm Figure 3: a: color map of shortest JSD b: color map of the distance between tibia and the depth of highest HU value (Δtibia-MaxHU)

7 Analysis We performed the following three analyses. To investigate the normal distribution of SBD at talus, SBD was averaged across each depths. Furthermore, group comparisons on SBD between male and female were performed using unpaired t-test with threshold for statistical significance set at p=0.05. Then, talus surface was divided in 9 sections (Figure 4). The shortest JSD, shortest Δtibia- MaxHU and highest HU section were selected. medial Ⅰ Ⅳ anterior Ⅱ Ⅴ Ⅲ Ⅵ Ⅶ Ⅷ Ⅸ posterior lateral Figure 4: Nine sections of talus surface Ⅰ antero-medial Ⅱ antero-middle Ⅲ antero-lateral Ⅳ middle-medial Ⅴ middle-middle Ⅵ middle-lateral Ⅶ postero-medial Ⅷ postero-middle Ⅸ postero-lateral

8 Results HU values HU values for ankle surface without weightbearing were following scores at each depth from talus joint surface (Figure 5). density(hu) ± ± ± ± ±79 313± ± mm 0.5mm 1.0mm 1.5mm 2.0mm 2.5mm 3.0mm depth Figure 5: HU values

9 Gender differences of HU values From 1.5mm to 3.0mm depth, the HU values of male was significant higher than female (Figure 6). density(hu) * * * * (*:p<0.05) 600 Male 400 Female mm 0.5mm 1.0mm 1.5mm 2.0mm 2.5mm 3.0mm depth Figure 6: Gender differences of HU values

10 Distribution of JSD, Δtibia-MaxHU, and HU values Each parameters were distributed medial sections (Table 1). Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ Ⅶ Ⅷ Ⅸ JSD Δtibia-MaxHU HU Table1 Distribution of JSD, Δtibia-MaxHU and HU values

11 Discussion Our study with normal subjects showed load stress distribution of talus joint surface. We observed that the area of maximum density for subchondral bone across the articular surface of the tibia was located in the medial part in most subjects. Therefore we observed that shortest joint space was located medial sections. We believed our data shows that stress concentration was increased in area of joint space narrowing especially. Also we observed gender differences of SBD. We considered the involvement of subject s build and body weight. CT-OAM analysis may elucidate the pathology of foot and ankle disorders as flatfoot and OA.

12 Conclusions We investigated the distribution of joint space distance and SBD. We observed that the area of maximum SBD and minimum JSD was located in the medial part of talus surface. CT-OAM analysis may elucidate the pathology of foot and ankle disorders. References [1] Müller-Gerbl M, et al. Skeletal Radiol 1989;18: [2] Müller-Gerbl M, et al. Clin Biomech 1990;5: [3] Muhlhofer H, et al. Surg Radiol Anat 2009;31:237-24