Clarke, Jon V and Riches, Philip E and Picard, Frederic and Deakin, Angela H

34 Pages·2017·0.32 MB·English

Computer Aided Surgery
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NON-INVASIVE COMPUTER-ASSISTED MEASUREMENT OF
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KNEE ALIGNMENT
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Journal: Computer Aided Surgery
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Manuscript ID: TCAS-2011-0014
Manuscript Type: Original Paper e
Date Submitted by the v
11-Mar-2011
Author: i
Complete List of Authors: Clarke, Jon; Golden Jubielee National Hospital, Orthopaedics;
University of Strathclyde, Department of Bioengineering
Riches, Philip; University of Strathclyde, Department of
Bioengineering w
Picard, Frederic; Golden Jubilee N ational Hospital, Orthopaedics
Deakin, Angela; Golden Jubilee National Hospital, Orthopaedics
Keywords: knee alignment, infrared tracking, nOon-invasive, computer-assisted

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Page 1 of 32 Computer Aided Surgery
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Title page
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7 NON-INVASIVE COMPUTER-ASSISTED MEASUREMENT OF KNEE
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ALIGNMENT
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12 Jon V. Clarke (MRCS)a,b, Philip E. Riches (PhD)a, Frederic Picard (MD)b, Angela H.
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14 Deakin (PhD)a,b
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a Department of Bioengineering, University of Strathclyde, Glasgow, Scotland
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19 b Department of Ort hopaedics, Golden Jubilee National Hospital, Clydebank, Scotland
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23 Running title: Non-invasieve measurement of knee alignment
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Corresponding Author:

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28 Mr Jon Clarke
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30 Clinical Research Fellow
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Department of Orthopaedics
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Golden Jubilee National Hospital i
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39 East Dunbartonshire
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G81 4DY
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Tel: [phone]
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46 Email: [email protected] y
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Computer Aided Surgery Page 2 of 32
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Abstract
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The quantification of knee alignment is a routine part of orthopaedic practice and is
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8 important for monitoring disease progression, planning interventional strategies and
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follow-up of patients. Currently available technologies such as radiographic
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13 measurements have a number of drawbacks. The aim of this study was to validate a
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15 potentially improved technique of measuring knee alignment under different conditions.
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An image-free navigation system was adapted for non-invasive use through the
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20 development of external infra-red tracker mountings. Stability was assessed by
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22 comparing the variance (F Test) of repeated mechanical femoro-tibial (MFT) angle
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measurements for a volunteeer and a leg model. MFT angles were then measured supine,
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27 standing and with varus-valgus stress for asymptomatic volunteers who each had two
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separate registrations and repeated measurements for each condition. The mean
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difference and 95% limits of agreement were used to assess intra-registration and inter-
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34 registration repeatability. For multiple registrations the range of measurements for the
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external mountings was 1° larger than the rigid model with statistically similar variance
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39 (p=0.34). Thirty volunteers were assessed (19 males, 11 f emales) with mean age 41 years
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41 (20-65) and mean BMI 26 (19-34). For intra-registratiOon repeatability, consecutive
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coronal alignment readings agreed to almost ±1° with up to ±0.5° loss of repeatability for
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46 coronal alignment measured before and after stress manoeuvres andy a ±0.2° following
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stance. Sagittal alignment measurements were less repeatable overall by an approximate
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factor of two
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53 Inter-registration agreement limits for coronal and sagittal supine MFT angles were ±1.6°
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and ±2.3° respectively. Varus and valgus stress measurements agreed to within ±1.3° and
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Page 3 of 32 Computer Aided Surgery
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±1.1° respectively. Agreement limits for standing MFT angles were ±2.9° (coronal) and
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±5.0° (sagittal) which may have reflected a variation in stance between measurements.
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8 The system provided repeatable, real-time measurements of coronal and sagittal knee
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alignment under a number of dynamic, real-time conditions offering a potential
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13 alternative to radiographs.
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17 Key words: knee alignment, non-invasive, infrared tracking, computer-assisted
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Computer Aided Surgery Page 4 of 32
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Introduction
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Knee joint alignment is an important parameter that has been extensively investigated in
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8 the context of osteoarthritis (OA). Radiographic and magnetic resonance imaging (MRI)
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studies have provided evidence that coronal malalignment is associated with an increased
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13 incidence [1] of tibiofemoral OA and risk of progression [2-5]. The importance of
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15 coronal alignment in reconstructive surgery of the knee has been widely accepted with
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the recognition that malpositioning can lead to early prosthesis loosening [6], with
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20 reported failure rates of 67% for varus knee prostheses versus 29% for knee prostheses in
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22 a neutral position [7], together with increased polyethylene wear and poor overall
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function [8,9]. Accurate meeasurement of knee alignment is therefore important for the
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27 monitoring of patients with OA, the subsequent planning of surgical interventions and the
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assessment of treatment outcomes.
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34 The standard measurement of knee alignment often relies on clinical evaluation in
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conjunction with radiographs that centre on the knee joint. However, human assessment
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39 of angles is known to be poor [10] and the accuracy of a lignment estimates under these
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41 circumstances may be no better than the order of ±5° [11]O. The use of knee radiographs
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has been found to be an inaccurate measure of mechanical lower limb alignment [12] and
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46 so its role in assessing knee alignment for planning intervention straytegies and for post-
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operative evaluation may be limited. Full-length hip-knee-ankle radiographs have
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therefore been increasingly adopted to provide more reliable pre- and post-operative
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53 information and are widely considered the gold standard for measuring knee alignment.
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In spite of enabling measurement of the mechanical femoro-tibial (MFT) angle these
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radiographs are susceptible to limb positioning errors with apparent variations in
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alignment produced as a result of knee flexion or rotation [13,14]. Computed tomography
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8 (CT) imaging can overcome these positional artefacts by providing a 3D evaluation of
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lower limb anatomy but is unable to provide weight-bearing information as subjects are
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13 required to be supine. Further drawbacks of both imaging modalities include limited
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15 availability, exposure of the pelvis to ionising radiation and the lack of more normal
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physiological control data from populations not typically exposed to them such as
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20 children and non-arthritic subjects with knee ligament injuries.
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Due to the limitations of radeiographs and CT scans, several alternative clinical measures
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27 of alignment have been reported and include techniques ranging from direct visual
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estimation to measurement adjuncts such as callipers, manual goniometers and plumb-
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line methods [15,16]. These methods are inexpensive, avoid radiation exposure and are
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34 relatively quick to perform with instant measurement results. However the reported errors
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are potentially too large for use in planning and follow-up of surgical interventions such
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39 as replacement arthroplasty and corrective osteotomy whe re higher levels of accuracy are
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46 Out with the clinic situation a number of new technologies using infyrared tracking have
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been introduced intra-operatively to provide surgeons with quantitative measurement
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tools that permit real time assessment of lower limb kinematics [17-19]. These systems
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53 have high levels of precision and can achieve angular and tibiofemoral gap measurements
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Computer Aided Surgery Page 6 of 32
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techniques have restricted scope due to their reliance on the rigid bony fixation of
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trackers. Adapting this technology for non-invasive patient assessment is challenging due
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8 to the soft tissue artefacts associated with the external mounting of trackers. Previous
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investigations to quantify the relative movement of external marker sets relative to
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13 underlying bones have reported large potential errors and questioned the value of these
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15 methods Ffor accurate kinematic analysis [22,23]. However these functional methods of
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determining rotational joint centres and resultant mechanical lower limb alignment are
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20 often in the context of gait analysis or involve active joint movement with contraction of
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22 the underlying muscles. A more recent study sought to minimise [24] these potential
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artefacts by measuring statiec standing lower limb alignment with position capture and
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27 skin markers along with external anatomical landmarks. The reliance on anthropometric
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measurements to predict joint centre location may have accounted for only a moderate
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correlation with corresponding long-leg radiographs in an experimental set-up not readily
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39 Given the subjective nature of clinical examination a nd the limitations of different
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41 measurement techniques reported to date, there is potential Oto improve current methods of
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assessing knee joint alignment. This paper reports the validation of a non-invasive system
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46 for measuring lower limb alignment based on a commercially availabyle infrared tracking
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technology with kinematic registration. Our hypothesis was that repeatable, real-time
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measurements of mechanical knee alignment under a number of conditions could be
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Page 7 of 32 Computer Aided Surgery
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Materials and Methods
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Infra-red tracking system
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8 An image-free navigation system (Orthopilot®, BBraun Aesculap, Tuttlingen, Germany),
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that consisted of an optical localiser, active infrared (IR) trackers, a pre-calibrated probe
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13 to digitise anatomical landmarks and a foot pedal that enabled ‘hands-free’ data recording
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15 was chosen due to its current clinical use. High tibial osteotomy (HTO) software
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(Orthopilot® HTO version 1.5, BBraun Aesculap, Tuttlingen, Germany) was used for the
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20 kinematic determination of hip, knee and ankle centres and resultant generation of
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22 coronal and sagittal MFT angles. Coronal alignment was defined with varus negative and
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valgus positive, whilst sagitteal alignment was defined with hyperextension negative and
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27 flexion positive.
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Rigid tracker mounting model
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34 A metal lower limb model was designed and manufactured to provide optimum
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conditions for measuring knee alignment. This consisted of metal rods representing a
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39 femur, tibia and a foot with rigidly attached tracker moun ts and mechanical hip, knee and
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centres (Figure 1).
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Non-invasive tracker mounting
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Tracker mountings for the thigh, calf and mid-foot regions were developed using metal
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Computer Aided Surgery Page 8 of 32
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Accessories, UK). A variety of lengths were made with a sequence of eyelets at either
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end to connect to the base plate and enable further adjustment of strap size (Figure 2).
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Tracker stability testing
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13 In order to quantify the soft tissue artefacts of the non-invasive mountings, the
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15 repeatability of the measurement of coronal knee alignment for both the leg model and
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for the right lower limb of a slim, female volunteer was determined. The volunteer was
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20 asked to relax whilst lying supine on an examination couch to ensure that all movements
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22 were passive. The registration process followed that which would be employed intra-
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operatively in the normal uese of the software. It began with the identification of the
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27 kinematic centre of the hip joint which required a slow, controlled circumduction of the
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thigh. The manoeuvre was performed in this manner to avoid moving the pelvis and
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subsequently altering the location of the rotational centre of the femoral head. If there
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34 was excessive movement of the pelvis or the trackers, then this could have resulted in a
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wider, “non-spherical” spread of acquired hip joint centre (HJC) points that was out with
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39 the required precision of the system [25]. This would result in rejection of the HJC
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41 acquisition and the instruction to repeat the circumduction Omanoeuvre until the spread of
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measured points was within the required threshold. The kinematic ankle centre was
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46 determined next by attaching a tracker to the dorsum of the foot anyd then dorsi-flexing
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and plantar-flexing the ankle. The rotational centre of the knee joint was then acquired by
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flexing and extending the knee between 0 and 90° as well as rotating the tibia on the
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53 femur at 90° of flexion. Following a single registration the trackers were left in position
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and 20 consecutive MFT angle recordings were made with the rigid leg model stationary
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Page 9 of 32 Computer Aided Surgery
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and with the volunteer instructed to remain as still as possible. The full registration
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process was then repeated a further 20 times on 13 different days to quantify additional
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8 soft tissue artefacts associated with removal and re-attachment of the trackers. Statistical
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analysis was performed using SPSS version 17 (SPSS Inc, Chicago, IL, USA) and F tests
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13 used for comparison of the variances of the repeated data sets
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Repeatability testing
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20 All experimental procedures were approved by the University Ethics Committee and,
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22 after giving informed consent, 30 volunteers were recruited (19 males and 11 females)
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with a mean age of 41 yeares (range 20-65) and a mean body mass index (BMI) of 26
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27 (range 19-34). Participants confirmed no acute knee symptoms and no history of joint
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replacement. Basic demographic data were recorded prior to assessment of the right
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lower limb. Two kinematic registration processes were performed using the appropriate
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34 passive clinical manoeuvres described above. After each registration, the immediate
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coronal and sagittal alignments in full extension were recorded with the lower limb
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39 supported at the heel and the subject told to relax. Follo wing this, coronal and sagittal
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41 alignment was measured with subjects asked to assumeO their normal bipedal stance.
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Returning the participant to the supine position, the coronal and sagittal alignment
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46 measurements were then performed twice and subsequent to this fyive manual stresses
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were applied to the knee joint by a single clinician to determine varus and valgus angular
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displacements. During these stress manaouevres, the knee was held between 0° and 5° of
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53 flexion as indicated by the on-screen measurement of sagittal MFT angle. If the knee
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coud not extend to 0° then the stress measurments were performed within a 5° window of
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