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PAPER
Comparing Norberg angle, linear femoral overlap and surface femoral overlap in radiographic assessment of the canine hip joint L. Janssens*, M. De Ridder*, G. Verhoeven†, I. Gielen† and H. van Bree† *Companion Animal Clinic KSD, Rotterdam, The Netherlands †School of Veterinary Medicine, Faculty of Medical Imaging, University of Ghent, Ghent, Belgium
OBJECTIVES: The aim of this study was to compare Norberg angle, linear femoral overlap and surface femoral overlap in radiographs of hips accepted for official hip dysplasia screening. METHODS: Radiographs of 100 dogs (200 hips) were reviewed. The three measurements were performed in each hip according to previously described methods and thereafter correlated. RESULTS: Intra- and inter-observer reliability was very high (all >85%). The linear correlation between Norberg angle and linear femoral overlap or surface femoral overlap was 0·40 and 0·41, respectively. The linear correlation between linear femoral overlap and surface femoral overlap was 0·84. CLINICAL SIGNIFICANCE: Norberg angle cannot be compared with surface femoral overlap or linear femoral overlap, and a mathematical formula could not be computed for the comparison.
Journal of Small Animal Practice (2013) 55, 135–138 DOI: 10.1111/jsap.12171 Accepted: 10 November 2013; Published online: 24 December 2013
INTRODUCTION Various measurement methods have been used in the radiographic diagnosis of canine hip dysplasia (CHD) and hip joint laxity, and in evaluation of the hip joint before and after surgical procedures such as triple pelvic osteotomy (TPO) (McLaughlin & Miler 1991, Dejardin et al. 1998, Tomlinson & Cook 2002). Common measurement methods include the Norberg angle or index (NA), linear femoral overlap (LFO) (McLaughlin & Miler 1991), surface femoral overlap (SFO) (Tomlinson & Johnson 2000, Tomlinson & Cook 2002), PennHIP distraction index (Smith et al. 1990) and dorsal acetabular rim angle (Slocum & Devine 1990). In the literature on TPO, the coverage of the femoral head before and after surgery is measured using NA, LFO and SFO (Dejardin et al. 1998). FO measurements in TPO cases are no doubt a better and more logical measuring method than NA measurements (Tomlinson & Cook 2002, Vandekerckhove et al. 2003). Indeed, the cranial acetabular rim, which is an important radiographic landmark in the NA measurement, is rotated/displaced ventrally after TPO surgery. The radiographic projection is not identical Journal of Small Animal Practice
•
Vol 55
•
March 2014
•
to the presurgical situation, which makes it difficult and even impossible to validate the difference in results of NA between presurgical and postsurgical radiographs. The aim of this study was to correlate NA, LFO and SFO. It was hypothesised that the three methods were comparable in describing the amount of femoral head coverage or hip coverage and that one value could be “translated” to the other two values through the use of comparison tables or a formula.
MATERIALS AND METHODS From the database of the Belgian National Committee for Inherited Skeletal Disorders (NCISD), 100 standard hipextended radiographs (200 hips) were selected at random, irrespective of the official hip dysplasia score. All radiographs fulfilled the technical criteria for official screening by the Federation Canine Internationale (FCI). The radiographs were made between 1996 and 2005 on 30 cm×40 cm cellulose films. It was unknown whether the radiographs were taken with or without sedation or anaesthesia. The median age of the dogs was 17 months (range, 10 to 61 months) and included 42 males and 58 females. Weights were
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not recorded. The selection of radiographs contained 25 breeds. These included German shepherd (n=24), Rottweiler (n=15), Dobermann (n=13), Newfoundland (n=8), Bernese mountain dog (n=5), Briard (n=3), golden retriever (n=3), Irish setter (n=3), Labrador retriever (n=3), Leonberger (3), Alaskan malamute (n=2), curly coated retriever (n=2), Kuvasz (n=2), standard poodle (n=2), Belgian shepherd (n=2) and one each of Border collie, Cane Corso, Chesapeake Bay retriever, pointer, great Dane, greater Swiss mountain dog, Hovawart, Landseer, Neapolitan mastiff and black Russian terrier. NA and LFO were measured on the original cellulose films using a transparent goniometer with 1 mm marks according to a previously described method (McLaughlin & Miller 1991, Vandekerckhove et al. 2003, Verhoeven et al. 2012) (Fig 1). The SFO (Fig 1) was measured (Tomlinson & Cook 2002) on the digitised radiographs using a Canvas 11 software (ACD Systems of America) according to a previously described method. Radiographs were digitised using an HP PSC 1610 scanner. Radiographic interpretation of the three measurements was performed by one examiner (MDR), a resident of the European College of Veterinary Surgeons (ECVS). Another blinded and independent examiner (LJ), a Diplomate of the ECVS, scored 20 of the dogs at random and blindly selected radiographs from all available radiographs (40 hips) to test reproducibility (inter-observer reliability testing). Radiographs from 20 dogs (40 hips) picked at random were measured in a blinded manner by the first examiner for repeatability (intra-observer reliability testing). Different radiographs were selected to assess repeatability and reproducibility. Statistical analysis Pearson’s correlation test was used to assess linear correlation between NA, LFO and SFO values. Intra- and inter-observer reliability was estimated by the ANOVA test. The Fisher’s exact test was used to define statistically significant difference between percentages. A significant difference was set at P105° for NA and >50% for FO (Culp et al. 2006, Doskarova et al. 2010, Skurkova et al. 2010, Smith et al. 2012). Percentages were calculated for each quadrant. In addition, the percentage of hips that would be considered normal with the NA method and normal or abnormal with the overlap methods was calculated.
RESULTS The median (range) NA was 107° (89° to 120°), the median LFO was 52% (30 to 67%) and the median SFO was 54% (26 to 70%). The intra-observer reliability was 98% for NA, 98% for LFO and 86% for SFO. The inter-observer reliability was 94% for NA, 92% for LFO and 86% for SFO. The linear correlation was 0·40 (P
PAPER
Comparing Norberg angle, linear femoral overlap and surface femoral overlap in radiographic assessment of the canine hip joint L. Janssens*, M. De Ridder*, G. Verhoeven†, I. Gielen† and H. van Bree† *Companion Animal Clinic KSD, Rotterdam, The Netherlands †School of Veterinary Medicine, Faculty of Medical Imaging, University of Ghent, Ghent, Belgium
OBJECTIVES: The aim of this study was to compare Norberg angle, linear femoral overlap and surface femoral overlap in radiographs of hips accepted for official hip dysplasia screening. METHODS: Radiographs of 100 dogs (200 hips) were reviewed. The three measurements were performed in each hip according to previously described methods and thereafter correlated. RESULTS: Intra- and inter-observer reliability was very high (all >85%). The linear correlation between Norberg angle and linear femoral overlap or surface femoral overlap was 0·40 and 0·41, respectively. The linear correlation between linear femoral overlap and surface femoral overlap was 0·84. CLINICAL SIGNIFICANCE: Norberg angle cannot be compared with surface femoral overlap or linear femoral overlap, and a mathematical formula could not be computed for the comparison.
Journal of Small Animal Practice (2013) 55, 135–138 DOI: 10.1111/jsap.12171 Accepted: 10 November 2013; Published online: 24 December 2013
INTRODUCTION Various measurement methods have been used in the radiographic diagnosis of canine hip dysplasia (CHD) and hip joint laxity, and in evaluation of the hip joint before and after surgical procedures such as triple pelvic osteotomy (TPO) (McLaughlin & Miler 1991, Dejardin et al. 1998, Tomlinson & Cook 2002). Common measurement methods include the Norberg angle or index (NA), linear femoral overlap (LFO) (McLaughlin & Miler 1991), surface femoral overlap (SFO) (Tomlinson & Johnson 2000, Tomlinson & Cook 2002), PennHIP distraction index (Smith et al. 1990) and dorsal acetabular rim angle (Slocum & Devine 1990). In the literature on TPO, the coverage of the femoral head before and after surgery is measured using NA, LFO and SFO (Dejardin et al. 1998). FO measurements in TPO cases are no doubt a better and more logical measuring method than NA measurements (Tomlinson & Cook 2002, Vandekerckhove et al. 2003). Indeed, the cranial acetabular rim, which is an important radiographic landmark in the NA measurement, is rotated/displaced ventrally after TPO surgery. The radiographic projection is not identical Journal of Small Animal Practice
•
Vol 55
•
March 2014
•
to the presurgical situation, which makes it difficult and even impossible to validate the difference in results of NA between presurgical and postsurgical radiographs. The aim of this study was to correlate NA, LFO and SFO. It was hypothesised that the three methods were comparable in describing the amount of femoral head coverage or hip coverage and that one value could be “translated” to the other two values through the use of comparison tables or a formula.
MATERIALS AND METHODS From the database of the Belgian National Committee for Inherited Skeletal Disorders (NCISD), 100 standard hipextended radiographs (200 hips) were selected at random, irrespective of the official hip dysplasia score. All radiographs fulfilled the technical criteria for official screening by the Federation Canine Internationale (FCI). The radiographs were made between 1996 and 2005 on 30 cm×40 cm cellulose films. It was unknown whether the radiographs were taken with or without sedation or anaesthesia. The median age of the dogs was 17 months (range, 10 to 61 months) and included 42 males and 58 females. Weights were
© 2013 British Small Animal Veterinary Association
135
L. Janssens et al.
not recorded. The selection of radiographs contained 25 breeds. These included German shepherd (n=24), Rottweiler (n=15), Dobermann (n=13), Newfoundland (n=8), Bernese mountain dog (n=5), Briard (n=3), golden retriever (n=3), Irish setter (n=3), Labrador retriever (n=3), Leonberger (3), Alaskan malamute (n=2), curly coated retriever (n=2), Kuvasz (n=2), standard poodle (n=2), Belgian shepherd (n=2) and one each of Border collie, Cane Corso, Chesapeake Bay retriever, pointer, great Dane, greater Swiss mountain dog, Hovawart, Landseer, Neapolitan mastiff and black Russian terrier. NA and LFO were measured on the original cellulose films using a transparent goniometer with 1 mm marks according to a previously described method (McLaughlin & Miller 1991, Vandekerckhove et al. 2003, Verhoeven et al. 2012) (Fig 1). The SFO (Fig 1) was measured (Tomlinson & Cook 2002) on the digitised radiographs using a Canvas 11 software (ACD Systems of America) according to a previously described method. Radiographs were digitised using an HP PSC 1610 scanner. Radiographic interpretation of the three measurements was performed by one examiner (MDR), a resident of the European College of Veterinary Surgeons (ECVS). Another blinded and independent examiner (LJ), a Diplomate of the ECVS, scored 20 of the dogs at random and blindly selected radiographs from all available radiographs (40 hips) to test reproducibility (inter-observer reliability testing). Radiographs from 20 dogs (40 hips) picked at random were measured in a blinded manner by the first examiner for repeatability (intra-observer reliability testing). Different radiographs were selected to assess repeatability and reproducibility. Statistical analysis Pearson’s correlation test was used to assess linear correlation between NA, LFO and SFO values. Intra- and inter-observer reliability was estimated by the ANOVA test. The Fisher’s exact test was used to define statistically significant difference between percentages. A significant difference was set at P105° for NA and >50% for FO (Culp et al. 2006, Doskarova et al. 2010, Skurkova et al. 2010, Smith et al. 2012). Percentages were calculated for each quadrant. In addition, the percentage of hips that would be considered normal with the NA method and normal or abnormal with the overlap methods was calculated.
RESULTS The median (range) NA was 107° (89° to 120°), the median LFO was 52% (30 to 67%) and the median SFO was 54% (26 to 70%). The intra-observer reliability was 98% for NA, 98% for LFO and 86% for SFO. The inter-observer reliability was 94% for NA, 92% for LFO and 86% for SFO. The linear correlation was 0·40 (P
