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Shameem Sampath: The association between shape descriptors of segmented areas of subchondral sclerosis of the upper tibia and the mechanical axis of the lower limb

Abstract

Introduction. Wolff’s law ( 1892) [1] proposes that the trabeculae in bone align with the orientation of dominant compressive loads within a bone. Previous studies have correlated trabeculae patterns with actual or theoretical patterns of loading in humans and in animals. Sclerosis of subchondral bone is a well recognised radiographic and surgical feature of osteoarthritic knees. Image analysis of the knee joint has quantified differences in bone texture on the medial and lateral side in patients with osteoarthritis. However, to the best of our knowledge, no previous work has been done to examine the relationship between such changes and the mechanical axis of the knee. In the United Kingdom, it is unusual for the Imaging Departments of hospitals to do long leg alignment views. Measurement of alignment preoperatively and postoperatively therefore is usually not done. It has been shown that the degree of preoperative malalignment determines the complexity of the total knee replacement (TKR), the duration of the procedure [2], and the choice of implant .Any system that can provide an accurate, reliable measurement of the long leg alignment using the standard antero posterior radiographs of the knee would therefore be a welcome addition at the planning stage. TKR surgery provides the opportunity both to examine radiographs of the knee and to measure the angle of the load line accurately with Navigation.

Aim. The aim of this study was to determine if a significant relationship exists between objective shape descriptors of the medial and lateral subchondral plates and the angle of the load line of the lower limb.

Materials and methods. This initial study was prospective and consisted of 27 consecutive patients.

Exclusion: Patients with previous surgery or known trauma to the bone of the proximal tibial , metabolic bone disease including Pagets, patients known to have been on oral or intravenous steroid medication, avascular necrosis, sickle cell disease, neoplasia, benign cysts, bone sclerosing or lytic conditions.

AP digital radiographs were taken of 27 osteoarthritic knees of patients booked for computer assisted total knee replacements (TKRs). The radiographs were taken with the patients standing in a standard position. ImageJ software from the National Institute of Health, USA, was used for image analysis. Each image was cropped proximally at the tip of the tibial spines and distally at the level of the lateral extent of the physeal scar. This meant that the proximal tip of the fibula was included in the cropped version. This was felt to be important to help with medial/lateral orientation during the later image processing. The medial and lateral limits were the edges of the tibial plateau including osteophytes.The cropped image was then thresholded and segmented. Thesholding was done to include only the maximum grey scale values in the chosen segments. The two regions of interest (ROI) in each image that comprised the medial and lateral segments corresponded to the areas of maximum sclerosis in medial and the lateral subchondral plates respectively. The shape determinants measured for each segment was the area, lengths of the major and minor axes, angle of the major axis, circularity, Feret’s diameter and Feret’s angle.

The navigated TKRs were performed using the Orthopilot navigation system V 4.2 (BBraun Aesculap, Tuttlingen) and the load line was determined. Statistical analysis was done using SPSS 18.

Results.

Of the 27 knees, 22 were varus, 4 were valgus and one had a neutral alignment.

The knees were analysed as three groups. Group 1 comprised all knees. Group 2 comprised the valgus knees. Group 3 comprised the neutral knee and the varus knees.

In Group 1, there were significant differences (p<0.05) between the area, length of the major axis, Ferret's diameter of the medial segment compared with the lateral segment.

There was a significant positive correlation (p<0.05) between the angle of the load line and the area of the medial subchondral plate in all knees. This relationship was stronger for varus knees, than for all knees.

Group 2, the valgus knees were too small a group for separate statistical analysis.

Group 3: The neutral and varus knees were then analysed as a separate group.

There was a statistically significant relationship between the angle of the load line measured by the Orthopilot at surgery, and the roundness and the angle of the major axis of the medial segment.

This relationship is given by the equation

VARUS = 9.1 + 2*MRound/L Round + 0.10*M area /L area - 0.04*cMangle

where VARUS is the angle of the load line measured by Orthopilot V4.2

         MRound and L Round are the roundness of the medial and lateral segments 
         M area and L area are the areas of the medial and lateral segments 
         Mangle is the angle of the major axis of the medial segment 
         p=0.0004    R2 = 60.3

Discussion.

This is a preliminary investigation. The relationship between the roundness and long axis angle of of the medial subchondral plate and the mechanical axis is consistent with Wolff’s Law and clinical experience. Although this relationship explains 60.3% of the variability in the load line axis, it is not strong enough to be a substitute for the Gold Standard measurement of the mechanical axis which is the long leg alignment views. Analysis of lateral radiographs was not done. It is possible that further analysis using lateral radiographs, done in conjunction with the present study may give a better representation in 3D of the association between the line of transmission of force and the areas of subchondral sclerosis.

References

[1] Wolff, J. (1892). Das Gesetz der Transformation der Knochen. Berlin: Hirschwald Verlag. 
[2] Sampath SA, Davies H, Voon S.(2009). Experience curve compared with manufacturing processes for TKA. Orthopedics. Oct;32(10 Suppl):49-51.

Acknowledgements.

The senior author is grateful to Uta who got him interested in image analysis

Keywords

knee, Wolff's Law, mechanical axis, segmentation

Administrative data

Presenting author: Shameem A C Sampath
Organisation: The Bluespot Knee Clinic

co-authors: South H Voon

Howard Davies

Type: Poster (portrait)

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