Non-invasive assessment of soft-tissue artifact and its effect on knee joint kinematics during functional activity
Introduction
Accurate in vivo measurement of knee joint kinematics is important for the evaluation of different surgical techniques, treatment methods and implant designs and for the development and validation of computer models capable of simulating normal and pathological movement (Pandy, 2001; Fernandez et al., 2008). Three-dimensional (3D) motion analysis using skin markers is the most common method for measuring knee joint kinematics in vivo. The accuracy of this approach is determined mainly by errors associated with the non-rigid movement of the soft-tissue interface between the skin markers and the underlying bone, commonly referred to as soft-tissue artifact (STA).
Numerous studies have investigated thigh and shank STA for a variety of different motor tasks, such as walking, running and sit-to-stand (Cappozzo et al., 1996; Wretenberg et al., 1996; Fuller et al., 1997; Reinschmidt et al., 1997; Stagni et al., 2005; Benoit et al., 2006; Tsai et al., 2009). All of these studies have found STA to be greater for the thigh than for the shank, with STA errors reaching values as high as 50 mm.
It is also important to quantify the propagation of STA to the estimation of knee joint kinematics. Previous studies have most often used intrusive techniques for their analyses, such as bone pins (Fuller et al., 1997; Reinschmidt et al., 1997; Benoit et al., 2006), external fixators (Cappozzo et al., 1996) and percutaneous tracking devices (Holden et al., 1997; Manal et al., 2000) to quantify joint motion in vivo. Unfortunately, these devices can restrict the movement of the subject and alter the normal, unimpeded sliding of the soft tissues relative to the underlying bone. To overcome this problem, non-invasive methods such as magnetic resonance imaging (MRI) (Sangeux et al., 2006) and X-ray fluoroscopy (Stagni et al., 2005; Garling et al., 2007; Sudhoff et al., 2007) have been used to quantify joint motion in vivo. However, these studies have been associated with several limitations, such as: (a) capturing several static poses throughout an arc of motion rather than measuring a continuous dynamic motion (Sangeux et al., 2006; Sudhoff et al., 2007); (b) investigating a single motor task only (Sangeux et al., 2006; Garling et al., 2007; Sudhoff et al., 2007); and (c) using elderly subjects fitted with knee implants (Stagni et al., 2005; Garling et al., 2007). The use of a non-invasive method to quantify the effect of STA on the estimation of knee joint kinematics has not been conducted on a group of healthy young adults for functional motor tasks such as walking.
The aim of this study was twofold: first, to combine subject-specific, MRI-based, 3D bone models and X-ray fluoroscopy to quantify lower limb STA non-invasively in young healthy subjects during functional activity; and second, to determine the effect of STA on the calculation of knee joint kinematics.
Section snippets
MRI bone models
Four able-bodied adult males (age=30±3 yr, weight=71±7 kg, height=178±2 cm) with no history of lower limb musculoskeletal injury participated in this study. Images of each subject’s left lower limb, from the pelvis to the ankle joint, were acquired from a 3T Siemens MRI device using a T2 fat-suppressed sequence (TE=12 ms, RT=23 ms, NEX=1, coronal plane images, slice thickness=1 mm, no gaps). The outlines of the exterior cortical bones were segmented using 3D Doctor (Able Software Corp.). Surface
Results
The average STA of all subjects was larger for the thigh markers than for the shank markers across all tasks (Table 1). The largest amount of STA for the thigh occurred in the proximodistal direction during the step-up task (12.6±4.7 mm), whereas that for the shank occurred in the mediolateral direction during the open-chain knee flexion task (8.6±2.7 mm).
For the tasks that involved a large amount of knee flexion (open-chain knee flexion, step-up and walking), skin markers on the mid-anterior
Discussion
The aim of this study was firstly, to quantify lower limb STA in young healthy subjects during functional activity; and secondly, to determine the effect of STA on the calculation of knee joint kinematics. Subject-specific, MRI-based bone models were generated and used, in conjunction with X-ray images obtained from video fluoroscopy, to accurately measure 3D knee joint rotations during four functional tasks. Soft-tissue artifact for the thigh was found to be larger than that for the shank (
Conflict of Interest
The authors do not have any financial or personal relationships with other people or organizations that could inappropriately influence this manuscript.
Acknowledgments
This work was supported by the Australian Research Council under Discovery Project Grants DP0772838 and DP0878705 and by a VESKI Innovation Fellowship to M.G. Pandy.
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