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A comparative study between proximal radial morphology and the floating radial head prosthesis
A comparative study between proximal radial morphology and the floating radial head prosthesis Nebojsa Popovic, MD, PhD,a Julien Djekic, MD,b Roger Lemaire, MD,a and Philippe Gillet, MD, PhD,a Liège, Belgium
A morphometric study of the proximal radius was performed with computed tomography scanning in 51 healthy adults. These dimensions were then compared with those of a commercially available floating radial head prosthesis. Results were expressed as mean values, SD, and minimum and maximum values. The minimum and maximum diameters of the radial head were 21.9 ⫾ 1.9 mm and 22.9 ⫾ 1.9 mm, respectively. The minimum and maximum intramedullary diameters of the radial neck were 8.3 ⫾ 1.3 mm and 9.3 ⫾ 1.5 mm, respectively. The combined length of the radial head and neck was 22.47 ⫾ 2.84 mm. The implications for prosthetic design are as follows: the small floating cup (19 mm in diameter) is too small for the large majority of adults, the large floating cup (22 mm in diameter) is closer to the radial anatomy, the mean values are significantly different between male and female patients, and a single component would suffice for right and left elbows. (J Shoulder Elbow Surg 2005;14:433-440.)
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ractures of the radial head with dislocation of the elbow, currently classified as Mason type IV injuries, can be difficult to manage.7,15,17 The scarce literature on the subject is of little help in defining optimal treatment.8,9,12,13,16,20,21,24 Several authors have developed prosthetic radial head replacements to improve immediate valgus stability and to prevent proximal translation of the radius.3,4,10,19,22,23 Prosthetic replacement of the radial head was first proposed by Speed22 in 1941 using a ferrule cup over the neck of the radius. Since that time, the use of acrylic, silicone rubber, vitallium, and other metallic radial head prostheses has been reported, usually with disappointing results.3,4,23 From the Orthopaedic Department and Department of Medical Imaging, University Hospital, Sart-Tilman. Reprint requests: Nebojsa Popovic, MD, PhD, Orthopaedic Department, University Hospital, Sart-Tilman (B.35), B-4000 Liège, Belgium (E-mail: [email protected]). Copyright © 2005 by Journal of Shoulder and Elbow Surgery Board of Trustees. 1058-2746/2005/$30.00 doi:10.1016/j.jse.2004.10.012
Restoring normal anatomy is the goal of a hemiarthroplasty. Radial head prostheses must restore the functions of the radial head within the elbow joint. These include positioning, load-bearing, and stability. Gupta et al5 have pointed out that currently available prosthetic implants are nonanatomic and that their designs are apparently not derived from geometric dimensions of the radial head. In addition, none of these prostheses incorporate stem designs that conform to the intramedullary canal of the proximal radius. Beredjiklian et al1 measured various radial head dimensions using magnetic resonance imaging (MRI) and showed that currently available radial head prosthetic stem designs overestimate the intramedullary dimensions of the radial neck. There are few reported anthropometric studies of the proximal radius on which to base the design of these implants.2,5,6,11 Furthermore, the importance of using a radial head prosthesis that closely matches the size and shape of the natural structure has not been well established. The aim of this study was to determine the morphology of the proximal radius by use of computed tomography (CT) scan measurements. In addition, we compared these findings with the dimensions of the floating radial head prosthesis (Tornier, Saint-Ismier, France) to determine whether the size of this implant adequately matches the morphologic characteristics of the proximal radius. MATERIALS AND METHODS A morphometric study of the proximal radius was performed with a spiral CT scanner (Picker PQ 6000; Phillips, Eindhoven, The Netherlands) to measure the anatomic dimensions of the radial head, neck, and shaft in 51 healthy adults. There were 28 men and 23 women with a mean age of 35.3 years (range, 20-58 years). All CT scans were performed first with the elbow in full extension and the forearm in supination. This was followed by a lateral series performed in 90° of flexion. Spiral CT parameters included 3-mm collimation and 3-mm table feed (pitch, 1.0) for the anteroposterior acquisition and 2-mm collimation and 2-mm table feed (pitch, 1.0) for the lateral views. The images were reconstructed with 2- and 1-mm increments, respectively, for the anteroposterior and lateral
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Figure 1 CT scan anthropometric measurements of the proximal radius: maximum (T1) and minimum (T2) diameters of the radial head (a), maximum depth of the articular surface (T4) and the angle formed by the midline of the radial neck and the long axis of the proximal radial diaphysis (A1 (b), and maximum (C5) and minimum (C6) intramedullary diameter of the radial neck and maximum (C7) and minimum (C8) external diameters of the radial neck measured just above the bicipital tuberosity (c).
views. Reconstruction parameters included bone or standard reconstructions. The duration of the examination was approximately 20 minutes. Scanning was performed at 130 kV, 150 mA, with a 512 ⫻ 512 matrix (spiral joint program). The following parameters were measured from axial, sagittal, and coronal images: maximum (T1) and minimum (T2) diameters of the radial head, height of the radial head on its medial side (T3), maximum depth of the articular surface (T4), maximum (C5) and minimum (C6) intramedullary diameters of the radial neck immediately above the bicipital tuberosity, maximum (C7) and minimum (C8) external diameters of the radial neck, radial neck length (C9), maximum (D10) and minimum (D11) diameters of the medullary canal of the proximal radius diaphysis just below the bicipital tuberosity, the angle formed by the midline of the
radial neck and the long axis of the proximal radial diaphysis (A1), and the combined length of the radial head and neck (A2) (Figures 1 and 2). Between January 1994 and December 2002, 39 floating radial head prostheses (Tornier, Saint-Ismier, France) were implanted at the Orthopaedic Department of the University Hospital, Liège, Belgium. This model of the floating radial head prosthesis is designed to articulate both with the humeral condyle and with the radial notch of the ulna. The implant is in 2 parts: a radial head made of high-density polyethylene enclosed in a cobalt-chrome cup that articulates in a semiconstrained manner with the spherical end of a cemented intramedullary stem. The stem has a neck-shaft angle of 15°. The semiconstrained articulation thus created allows free rotation and a uniplanar arc of motion of 35° from any given point. The prosthesis is available with 2
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radial head diameters (19 and 22 mm); the height of the prosthetic head is 14 mm. The radial stem comes in 2 sizes; one has a diameter of 8 mm and a length of 60 mm, and the other has a diameter of 6.5 mm and a length of 55 mm. These components are interchangeable and allow 4 different combinations of head and stem. Descriptive statistical analysis was first performed, including global mean values, for each measured variable. Paired t test and signed rank test were used to compare values from the left and right sides and dominant and nondominant elbows for each measured variable. Finally, 2-sample t tests for each variable by sex and correlations between all morphometric variables were performed.
RESULTS Morphometric measurements of the proximal radius (mean ⫾ SD) are summarized in Figures 3 and 4. Figure 2 Height of the radial head and length of the radial neck measured on transverse CT slices. T3, Greatest height of the radial head, measured on its medial side from the proximal border of the radial head to its inferior border; C9, length of the radial neck made on transverse CT slide from the distal border of the medial side of the radial head to the proximal border of the bicipital tuberosity; A2, combined length of the radial head and neck.
Radial head dimensions
Histograms of most relevant morphometric variables of the radial head are presented in Figure 3. The maximum and minimum diameters of the radial
Figure 3 Histogram of the most relevant variables of the radial head. T1, Maximum diameter of the radial head; T2, minimum diameter of the radial head; T3, radial head height; T4, maximum depth of the articular surface of the radial head.
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Figure 4 Histogram of the most relevant variables of the radial neck and proximal shaft. C5 and C6, Maximum and minimum intramedullary diameters of the radial neck; C7 and C8, maximum and minimum external diameters of the radial neck; C9, radial neck length; D10 and D11, maximum and minimum diameters of the medullary canal of the proximal radius diaphysis.
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There was a significant difference between the maximum and minimum radial head diameters in men and women (Figure 7). The maximum diameter was 24.24 mm on average in men and 21.19 mm in women; the minimum diameter was 23.21 mm on average in men and 20.21 mm in women. DISCUSSION
Figure 5 Correlation of the radial head diameters of the left and right elbows.
head were 22.8 ⫾ 1.9 mm and 21.8 ⫾ 1.9 mm, respectively. The depth of the articular surface averaged 2.4 ⫾ 0.5 mm, and the mean radial head height was 9.9 ⫾ 1.6 mm. Radial neck and shaft dimensions
Histograms of the most relevant morphometric variables of the radial neck and shaft are presented in Figure 4. The maximum and minimum intramedullary diameters of the radial neck just above the bicipital tuberosity were 9.3 ⫾ 1.5 mm and 8.3 ⫾ 1.3 mm, respectively. The maximum and minimum external diameters of the radial neck were 15.1 ⫾ 1.49 mm and 13.7 ⫾ 1.3 mm, respectively, and the mean radial neck length was 12.3 ⫾ 2.81 mm. The maximum and minimum diameters of the medullary canal of the proximal radial diaphysis were 8.3 ⫾ 1.39 mm and 7.5 ⫾ 1.27 mm, respectively. The angle formed by the midline of the radial neck with the long axis of the proximal radial diaphysis was 15.16° ⫾ 4.15°. The combined length of the radial head and neck was 22.47 ⫾ 2.84 mm. The diameters of the left and right radial heads were highly correlated (Figure 5). There was no significant difference between left and right radial head diameters. The diameter of one side can therefore be substituted for measurement of the contralateral side. Of particular interest is the fact that the dimensions of the large floating cup (22 mm) approach the normal proximal radial anatomy whereas the small floating cup (19 mm) does not correspond to normal adult anatomy (Figure 3). The relationship between the radial head prostheses (19 mm and 22 mm) and the mean anthropometric values of the study group is shown in Figure 6.
Knowledge of the size and shape of the proximal radius is necessary in the design of a radial head prosthesis that is anatomically and biomechanically adequate. There are few anthropometric studies of the proximal radius published in the literature.1,6,11 Some studies were done on dry cadaveric bone2; others were done on fresh cadaveric bones.11 Some authors have attempted to describe the morphologic characteristics of the proximal radius on plain radiographs,11 and others have used MRI films.1 Our study shows a significant difference between the maximum diameter (22.8 mm) and the minimum diameter (21.8 mm) of the radial head, with a mean difference of 1.0 mm. The results are largely in agreement with published data. Captier et al2 have shown in a study on dried cadaveric bone that we need to distinguish 2 shapes of the radial head with regard to the difference between the maximum and the minimum diameter; in their study, 57% of radial heads were elliptical and 43% were circular. In a study on fresh cadaveric bones, King et al11 did not note any difference between the ventrodorsal diameter or the transverse diameter of the radial head whereas, on radiographs, the transverse diameter was greater than the anteroposterior diameter (24.3 mm and 23.0 mm, respectively). According to the results of our study, we agree with King et al11 that the radial head is consistently neither circular nor elliptic, as its diameters were found to be similar in some individuals and significantly different in others. Beredjiklian et al1 found a significant difference between radial head dimensions in men and women with respect to the maximum and minimum diameters. The results obtained from our study confirm that a difference exists between men and women. No differences were noted based on left or right side or age. A single head component should thus be sufficient for right and left elbows. The height of the radial head is very important from the biomechanical point of view and must be preserved to restore stability in both the elbow and the wrist.14 Captier et al2 found, in dry cadaveric bone, that the greatest height of the radial head is in its medial portion and measures 15.3 mm. This value is greater than that measured by elbow MRI in another study,1 which showed a 12-mm height in the frontal and sagittal planes. Our study shows a smaller value (11 mm). One possible explanation is that CT scan
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Figure 6 Comparison of the mean values of the study group and the floating radial head prosthesis (22 mm [LFC] and 19 mm [SFC]).
measurements do not take into account the thickness of the articular cartilage. The diameter of the intramedullary canal of the proximal radial diaphysis is relatively small. The results of our study are similar on this point to those in the studies by King et al11 and Beredjiklian et al.1 Our method of data collection was different. CT scan does not take into account the thickness of the articular cartilage, which may result in values slightly lower than those gained with MRI1 or direct measurement of fresh cadaveric bone.2 However, CT scan provided an accurate means by which to measure both the surface and intramedullary dimensions of the radial head and neck. The design of a radial head implant should be based on a detailed understanding of the anatomy and biomechanics of the elbow, including its motion and the forces applied to the joint surfaces. Even though the elbow is a non–weight-bearing joint, the forces applied to it are significant.18 A radial head replacement should be as close to the normal anatomy as possible, because of the requirement to balance valgus stress and the cyclic axial bending force on the ulnar component and to reduce the stress concentration on the ulnohumeral joint.12 The prosthesis must articulate with the capitellum and the ulna;
it is probable that the size and shape of the implant are important from the viewpoint of joint kinematics, stress in the implant, and load transfer. On the basis of our results, the small floating cup of the Judet prosthesis (19 mm) is too small for use in the vast majority of adults whereas the large floating cup (22 mm) better approximates the anatomy of the proximal radius. Our findings also confirm that one should measure the largest contralateral radial head diameter when choosing the diameter of the floating cup. Our findings demonstrate that the stems of the Judet floating radial head prostheses do not overestimate intramedullary dimensions. The stem has a diameter of 6 or 8 mm at the level of the bicipital tuberosity, and it has a conical shape. This makes fitting the stem into the radius easy but also means that we are unable to determine the degree of press-fit achieved in a specific patient. Some authors showed that there is a poor correlation between the diameter of the radial head and that of the canal,2,11 suggesting that some patients might require a large head and a small stem whereas others might require a small head and a large stem. Modular floating radial head prostheses with heads and stems packaged separately may obviate the large inventory that otherwise is required for a monoblock implant system. Further-
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Figure 7 Comparison of observed anthropometric values between men and women at different measurement sites.
more, the semiconstrained joint between the prosthetic head and cup improves contact between the radial head and the capitellum by possible adjustment through the bipolar articulation. The combined length of the bony radial head and neck in this study was 22.4 mm on average. The combined length of the prosthetic radial head and the neck of the prosthetic stem with the Judet prosthesis was 32.2 mm (14.7 ⫹ 17.5 mm) on average; during the fixation of the prosthetic neck into the prosthetic head, the neck protrudes into the head to a depth of 10 mm, which results in an overall length of 22.2 mm. The effect of minimal radial shortening on radiocapitellar mechanics and valgus stability of the elbow joint is unclear. Clinical experience may suggest that shortening by more than 3 mm could be significant in the overall functioning of the radiocapitellar joint. However, these assertions have not been confirmed in the literature. Our data from the anthropometric study of the proximal radial anatomy and data published in the literature suggest that the radial prostheses should be designed in 2 independent parts: the head and the intramedullary stem. The commercially available floating radial head prosthesis described in this study almost adequately matches the morphologic characteristics of the proximal intramedullary canal of the radius, but the small floating cup (19 mm) is too small for use in the vast majority of adults. However, be-
cause of large variations in normal anatomy, a perfectly conforming radial head prosthesis is practically impossible to design. Designing floating radial head prostheses by taking into account the anatomic dimensions and morphologic variability of the proximal radius may lead to improvement in function after radial head replacement. Further clinical investigation and long-term patient follow-up are required to evaluate this assertion. REFERENCES
1. Beredjiklian PD, Nalbantoglu V, Potter HG, Hotchkiss RN. Prosthetic radial components and proximal radial head morphology: a mismatch. J Shoulder Elbow Surg 1999;8:471-5. 2. Captier G, Canovas F, Mercier N, Thomas E, Bonnel F. Biometry of the radial head: biomechanical implications in pronation and supination. Surg Radiol Anat 2002;24:295-301. 3. Carr CR, Howard JW. Metallic cap replacement of the radial head following fracture. West J Surg Obstet Gynecol 1951;59: 539-46. 4. Cherry JC. Use of acrylic prosthesis in the treatment of fracture of the head of the radius. J Bone Joint Surg Br 1953;35:70-1. 5. Gupta GG, Lucas G, Hahn DL. Biomechanical and computer analysis of the radial head prostheses. J Shoulder Elbow Surg 1997;6:37-48. 6. Gupta GG, Moore-Jansen PH, Lucas GL. Differences in radial head dimensions based on gender, race, age and side. Orthop Trans 1992;16:342. 7. Harrington IJ, Sekyi-Otu A, Barrington TW, Evans DC, Tuli V. The functional outcome with metallic radial head implants in the treatment of unstable elbow fractures: a long-term review. J Trauma 2001;50:46-53.
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8. Harrington IJ, Tountas AA. Replacement of the radial head in the treatment of unstable elbow fractures. Injury 1981;12:405-12. 9. Holmenschlager F, Halm JP, Piatek S, Schubert S, Winckler S. Comminuted radial head fractures. Initial experiences with a Judet radial head prosthesis [in German]. Unfallchirurg 2002;105: 344-52. 10. Judet T, Garreau-Deloubresse C, Pirion P, Charnley G. A floating prosthesis for radial head fractures. J Bone Joint Surg Br 1996; 78:244-9. 11. King GJW, Zarzour ZDS, Pattersen SD, Johnson JA. An anthropometric study of the radial head. Implication in the design of a prosthesis. J Arthroplasty 2001;16:112-7. 12. King GJW, Zarzour ZDS, Dunning CD, Patterson SD, Johnson JA. Metallic radial head arthroplasty improves the valgus stability of the medial collateral deficient elbow. Clin Orthop 1999;368: 114-25. 13. Knight DJ, Rymaszewski LA, Amis AA, Miller JH. Primary replacement of a fractured radial head. J Bone Joint Surg Br 1993;75: 572-6. 14. Leppilathi J, Jalovaara P. Early excision of the radial head for fracture. Int Orthop 2001;24:160-2. 15. Mikic A, Vukadinovic S. Late results in fractures of the radial head treated by excision. Clin Orthop 1983;181:220-8. 16. Moro J, Werier J, MacDermid JC, Patterson SD, King GJ. Arthro-
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17. 18. 19. 20.
21. 22. 23. 24.
plasty with a metal radial head for unreconstructible fractures of the radial head. J Bone Joint Surg Am 2001;83:1201-11. Morrey BF. Complex instability of the elbow. J Bone Joint Surg Am 1997;79:460-9. Morrey BF, An KN, Stormont TJ. Force transmission through the radial head. J Bone Joint Surg Am 1988;70:250-6. Pomianowski S, Sawicki G, Orlowski J, et al. Bipolar radial head prosthesis— own design [in Polish]. Chir Narzadow Ruchu Ortop Pol 2003;68:53-7. Popovic N, Gillet P, Rodriguez A, Lemaire R. Fracture of the radial head with associated elbow dislocation: results of treatment using a floating radial head prosthesis. J Orthop Trauma 2000;14: 171-7. Smets S, Govaers K, Jansen N, et al. The floating radial head prosthesis for comminuted radial head fractures: a multicentric study. Acta Orthop Belg 2000;66:353-8. Speed K. Ferrule caps for the head of the radius. Surg Gynecol Obstet 1941;73:845-50. Swanson AB, Jaeger SH, La Rochelle D. Comminuted fractures of the radial head. The role of silicone implant replacement arthroplasty. J Bone Joint Surg Am 1981;63:1039-49. Van Glabbeek F, Van Riet R, Verstreken J. Current concepts in the treatment of radial head fractures in the adult. A clinical and biomechanical approach. Acta Orthop Belg 2001;67:340-1.
A morphometric study of the proximal radius was performed with computed tomography scanning in 51 healthy adults. These dimensions were then compared with those of a commercially available floating radial head prosthesis. Results were expressed as mean values, SD, and minimum and maximum values. The minimum and maximum diameters of the radial head were 21.9 ⫾ 1.9 mm and 22.9 ⫾ 1.9 mm, respectively. The minimum and maximum intramedullary diameters of the radial neck were 8.3 ⫾ 1.3 mm and 9.3 ⫾ 1.5 mm, respectively. The combined length of the radial head and neck was 22.47 ⫾ 2.84 mm. The implications for prosthetic design are as follows: the small floating cup (19 mm in diameter) is too small for the large majority of adults, the large floating cup (22 mm in diameter) is closer to the radial anatomy, the mean values are significantly different between male and female patients, and a single component would suffice for right and left elbows. (J Shoulder Elbow Surg 2005;14:433-440.)
F
ractures of the radial head with dislocation of the elbow, currently classified as Mason type IV injuries, can be difficult to manage.7,15,17 The scarce literature on the subject is of little help in defining optimal treatment.8,9,12,13,16,20,21,24 Several authors have developed prosthetic radial head replacements to improve immediate valgus stability and to prevent proximal translation of the radius.3,4,10,19,22,23 Prosthetic replacement of the radial head was first proposed by Speed22 in 1941 using a ferrule cup over the neck of the radius. Since that time, the use of acrylic, silicone rubber, vitallium, and other metallic radial head prostheses has been reported, usually with disappointing results.3,4,23 From the Orthopaedic Department and Department of Medical Imaging, University Hospital, Sart-Tilman. Reprint requests: Nebojsa Popovic, MD, PhD, Orthopaedic Department, University Hospital, Sart-Tilman (B.35), B-4000 Liège, Belgium (E-mail: [email protected]). Copyright © 2005 by Journal of Shoulder and Elbow Surgery Board of Trustees. 1058-2746/2005/$30.00 doi:10.1016/j.jse.2004.10.012
Restoring normal anatomy is the goal of a hemiarthroplasty. Radial head prostheses must restore the functions of the radial head within the elbow joint. These include positioning, load-bearing, and stability. Gupta et al5 have pointed out that currently available prosthetic implants are nonanatomic and that their designs are apparently not derived from geometric dimensions of the radial head. In addition, none of these prostheses incorporate stem designs that conform to the intramedullary canal of the proximal radius. Beredjiklian et al1 measured various radial head dimensions using magnetic resonance imaging (MRI) and showed that currently available radial head prosthetic stem designs overestimate the intramedullary dimensions of the radial neck. There are few reported anthropometric studies of the proximal radius on which to base the design of these implants.2,5,6,11 Furthermore, the importance of using a radial head prosthesis that closely matches the size and shape of the natural structure has not been well established. The aim of this study was to determine the morphology of the proximal radius by use of computed tomography (CT) scan measurements. In addition, we compared these findings with the dimensions of the floating radial head prosthesis (Tornier, Saint-Ismier, France) to determine whether the size of this implant adequately matches the morphologic characteristics of the proximal radius. MATERIALS AND METHODS A morphometric study of the proximal radius was performed with a spiral CT scanner (Picker PQ 6000; Phillips, Eindhoven, The Netherlands) to measure the anatomic dimensions of the radial head, neck, and shaft in 51 healthy adults. There were 28 men and 23 women with a mean age of 35.3 years (range, 20-58 years). All CT scans were performed first with the elbow in full extension and the forearm in supination. This was followed by a lateral series performed in 90° of flexion. Spiral CT parameters included 3-mm collimation and 3-mm table feed (pitch, 1.0) for the anteroposterior acquisition and 2-mm collimation and 2-mm table feed (pitch, 1.0) for the lateral views. The images were reconstructed with 2- and 1-mm increments, respectively, for the anteroposterior and lateral
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Figure 1 CT scan anthropometric measurements of the proximal radius: maximum (T1) and minimum (T2) diameters of the radial head (a), maximum depth of the articular surface (T4) and the angle formed by the midline of the radial neck and the long axis of the proximal radial diaphysis (A1 (b), and maximum (C5) and minimum (C6) intramedullary diameter of the radial neck and maximum (C7) and minimum (C8) external diameters of the radial neck measured just above the bicipital tuberosity (c).
views. Reconstruction parameters included bone or standard reconstructions. The duration of the examination was approximately 20 minutes. Scanning was performed at 130 kV, 150 mA, with a 512 ⫻ 512 matrix (spiral joint program). The following parameters were measured from axial, sagittal, and coronal images: maximum (T1) and minimum (T2) diameters of the radial head, height of the radial head on its medial side (T3), maximum depth of the articular surface (T4), maximum (C5) and minimum (C6) intramedullary diameters of the radial neck immediately above the bicipital tuberosity, maximum (C7) and minimum (C8) external diameters of the radial neck, radial neck length (C9), maximum (D10) and minimum (D11) diameters of the medullary canal of the proximal radius diaphysis just below the bicipital tuberosity, the angle formed by the midline of the
radial neck and the long axis of the proximal radial diaphysis (A1), and the combined length of the radial head and neck (A2) (Figures 1 and 2). Between January 1994 and December 2002, 39 floating radial head prostheses (Tornier, Saint-Ismier, France) were implanted at the Orthopaedic Department of the University Hospital, Liège, Belgium. This model of the floating radial head prosthesis is designed to articulate both with the humeral condyle and with the radial notch of the ulna. The implant is in 2 parts: a radial head made of high-density polyethylene enclosed in a cobalt-chrome cup that articulates in a semiconstrained manner with the spherical end of a cemented intramedullary stem. The stem has a neck-shaft angle of 15°. The semiconstrained articulation thus created allows free rotation and a uniplanar arc of motion of 35° from any given point. The prosthesis is available with 2
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radial head diameters (19 and 22 mm); the height of the prosthetic head is 14 mm. The radial stem comes in 2 sizes; one has a diameter of 8 mm and a length of 60 mm, and the other has a diameter of 6.5 mm and a length of 55 mm. These components are interchangeable and allow 4 different combinations of head and stem. Descriptive statistical analysis was first performed, including global mean values, for each measured variable. Paired t test and signed rank test were used to compare values from the left and right sides and dominant and nondominant elbows for each measured variable. Finally, 2-sample t tests for each variable by sex and correlations between all morphometric variables were performed.
RESULTS Morphometric measurements of the proximal radius (mean ⫾ SD) are summarized in Figures 3 and 4. Figure 2 Height of the radial head and length of the radial neck measured on transverse CT slices. T3, Greatest height of the radial head, measured on its medial side from the proximal border of the radial head to its inferior border; C9, length of the radial neck made on transverse CT slide from the distal border of the medial side of the radial head to the proximal border of the bicipital tuberosity; A2, combined length of the radial head and neck.
Radial head dimensions
Histograms of most relevant morphometric variables of the radial head are presented in Figure 3. The maximum and minimum diameters of the radial
Figure 3 Histogram of the most relevant variables of the radial head. T1, Maximum diameter of the radial head; T2, minimum diameter of the radial head; T3, radial head height; T4, maximum depth of the articular surface of the radial head.
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Figure 4 Histogram of the most relevant variables of the radial neck and proximal shaft. C5 and C6, Maximum and minimum intramedullary diameters of the radial neck; C7 and C8, maximum and minimum external diameters of the radial neck; C9, radial neck length; D10 and D11, maximum and minimum diameters of the medullary canal of the proximal radius diaphysis.
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There was a significant difference between the maximum and minimum radial head diameters in men and women (Figure 7). The maximum diameter was 24.24 mm on average in men and 21.19 mm in women; the minimum diameter was 23.21 mm on average in men and 20.21 mm in women. DISCUSSION
Figure 5 Correlation of the radial head diameters of the left and right elbows.
head were 22.8 ⫾ 1.9 mm and 21.8 ⫾ 1.9 mm, respectively. The depth of the articular surface averaged 2.4 ⫾ 0.5 mm, and the mean radial head height was 9.9 ⫾ 1.6 mm. Radial neck and shaft dimensions
Histograms of the most relevant morphometric variables of the radial neck and shaft are presented in Figure 4. The maximum and minimum intramedullary diameters of the radial neck just above the bicipital tuberosity were 9.3 ⫾ 1.5 mm and 8.3 ⫾ 1.3 mm, respectively. The maximum and minimum external diameters of the radial neck were 15.1 ⫾ 1.49 mm and 13.7 ⫾ 1.3 mm, respectively, and the mean radial neck length was 12.3 ⫾ 2.81 mm. The maximum and minimum diameters of the medullary canal of the proximal radial diaphysis were 8.3 ⫾ 1.39 mm and 7.5 ⫾ 1.27 mm, respectively. The angle formed by the midline of the radial neck with the long axis of the proximal radial diaphysis was 15.16° ⫾ 4.15°. The combined length of the radial head and neck was 22.47 ⫾ 2.84 mm. The diameters of the left and right radial heads were highly correlated (Figure 5). There was no significant difference between left and right radial head diameters. The diameter of one side can therefore be substituted for measurement of the contralateral side. Of particular interest is the fact that the dimensions of the large floating cup (22 mm) approach the normal proximal radial anatomy whereas the small floating cup (19 mm) does not correspond to normal adult anatomy (Figure 3). The relationship between the radial head prostheses (19 mm and 22 mm) and the mean anthropometric values of the study group is shown in Figure 6.
Knowledge of the size and shape of the proximal radius is necessary in the design of a radial head prosthesis that is anatomically and biomechanically adequate. There are few anthropometric studies of the proximal radius published in the literature.1,6,11 Some studies were done on dry cadaveric bone2; others were done on fresh cadaveric bones.11 Some authors have attempted to describe the morphologic characteristics of the proximal radius on plain radiographs,11 and others have used MRI films.1 Our study shows a significant difference between the maximum diameter (22.8 mm) and the minimum diameter (21.8 mm) of the radial head, with a mean difference of 1.0 mm. The results are largely in agreement with published data. Captier et al2 have shown in a study on dried cadaveric bone that we need to distinguish 2 shapes of the radial head with regard to the difference between the maximum and the minimum diameter; in their study, 57% of radial heads were elliptical and 43% were circular. In a study on fresh cadaveric bones, King et al11 did not note any difference between the ventrodorsal diameter or the transverse diameter of the radial head whereas, on radiographs, the transverse diameter was greater than the anteroposterior diameter (24.3 mm and 23.0 mm, respectively). According to the results of our study, we agree with King et al11 that the radial head is consistently neither circular nor elliptic, as its diameters were found to be similar in some individuals and significantly different in others. Beredjiklian et al1 found a significant difference between radial head dimensions in men and women with respect to the maximum and minimum diameters. The results obtained from our study confirm that a difference exists between men and women. No differences were noted based on left or right side or age. A single head component should thus be sufficient for right and left elbows. The height of the radial head is very important from the biomechanical point of view and must be preserved to restore stability in both the elbow and the wrist.14 Captier et al2 found, in dry cadaveric bone, that the greatest height of the radial head is in its medial portion and measures 15.3 mm. This value is greater than that measured by elbow MRI in another study,1 which showed a 12-mm height in the frontal and sagittal planes. Our study shows a smaller value (11 mm). One possible explanation is that CT scan
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Figure 6 Comparison of the mean values of the study group and the floating radial head prosthesis (22 mm [LFC] and 19 mm [SFC]).
measurements do not take into account the thickness of the articular cartilage. The diameter of the intramedullary canal of the proximal radial diaphysis is relatively small. The results of our study are similar on this point to those in the studies by King et al11 and Beredjiklian et al.1 Our method of data collection was different. CT scan does not take into account the thickness of the articular cartilage, which may result in values slightly lower than those gained with MRI1 or direct measurement of fresh cadaveric bone.2 However, CT scan provided an accurate means by which to measure both the surface and intramedullary dimensions of the radial head and neck. The design of a radial head implant should be based on a detailed understanding of the anatomy and biomechanics of the elbow, including its motion and the forces applied to the joint surfaces. Even though the elbow is a non–weight-bearing joint, the forces applied to it are significant.18 A radial head replacement should be as close to the normal anatomy as possible, because of the requirement to balance valgus stress and the cyclic axial bending force on the ulnar component and to reduce the stress concentration on the ulnohumeral joint.12 The prosthesis must articulate with the capitellum and the ulna;
it is probable that the size and shape of the implant are important from the viewpoint of joint kinematics, stress in the implant, and load transfer. On the basis of our results, the small floating cup of the Judet prosthesis (19 mm) is too small for use in the vast majority of adults whereas the large floating cup (22 mm) better approximates the anatomy of the proximal radius. Our findings also confirm that one should measure the largest contralateral radial head diameter when choosing the diameter of the floating cup. Our findings demonstrate that the stems of the Judet floating radial head prostheses do not overestimate intramedullary dimensions. The stem has a diameter of 6 or 8 mm at the level of the bicipital tuberosity, and it has a conical shape. This makes fitting the stem into the radius easy but also means that we are unable to determine the degree of press-fit achieved in a specific patient. Some authors showed that there is a poor correlation between the diameter of the radial head and that of the canal,2,11 suggesting that some patients might require a large head and a small stem whereas others might require a small head and a large stem. Modular floating radial head prostheses with heads and stems packaged separately may obviate the large inventory that otherwise is required for a monoblock implant system. Further-
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Figure 7 Comparison of observed anthropometric values between men and women at different measurement sites.
more, the semiconstrained joint between the prosthetic head and cup improves contact between the radial head and the capitellum by possible adjustment through the bipolar articulation. The combined length of the bony radial head and neck in this study was 22.4 mm on average. The combined length of the prosthetic radial head and the neck of the prosthetic stem with the Judet prosthesis was 32.2 mm (14.7 ⫹ 17.5 mm) on average; during the fixation of the prosthetic neck into the prosthetic head, the neck protrudes into the head to a depth of 10 mm, which results in an overall length of 22.2 mm. The effect of minimal radial shortening on radiocapitellar mechanics and valgus stability of the elbow joint is unclear. Clinical experience may suggest that shortening by more than 3 mm could be significant in the overall functioning of the radiocapitellar joint. However, these assertions have not been confirmed in the literature. Our data from the anthropometric study of the proximal radial anatomy and data published in the literature suggest that the radial prostheses should be designed in 2 independent parts: the head and the intramedullary stem. The commercially available floating radial head prosthesis described in this study almost adequately matches the morphologic characteristics of the proximal intramedullary canal of the radius, but the small floating cup (19 mm) is too small for use in the vast majority of adults. However, be-
cause of large variations in normal anatomy, a perfectly conforming radial head prosthesis is practically impossible to design. Designing floating radial head prostheses by taking into account the anatomic dimensions and morphologic variability of the proximal radius may lead to improvement in function after radial head replacement. Further clinical investigation and long-term patient follow-up are required to evaluate this assertion. REFERENCES
1. Beredjiklian PD, Nalbantoglu V, Potter HG, Hotchkiss RN. Prosthetic radial components and proximal radial head morphology: a mismatch. J Shoulder Elbow Surg 1999;8:471-5. 2. Captier G, Canovas F, Mercier N, Thomas E, Bonnel F. Biometry of the radial head: biomechanical implications in pronation and supination. Surg Radiol Anat 2002;24:295-301. 3. Carr CR, Howard JW. Metallic cap replacement of the radial head following fracture. West J Surg Obstet Gynecol 1951;59: 539-46. 4. Cherry JC. Use of acrylic prosthesis in the treatment of fracture of the head of the radius. J Bone Joint Surg Br 1953;35:70-1. 5. Gupta GG, Lucas G, Hahn DL. Biomechanical and computer analysis of the radial head prostheses. J Shoulder Elbow Surg 1997;6:37-48. 6. Gupta GG, Moore-Jansen PH, Lucas GL. Differences in radial head dimensions based on gender, race, age and side. Orthop Trans 1992;16:342. 7. Harrington IJ, Sekyi-Otu A, Barrington TW, Evans DC, Tuli V. The functional outcome with metallic radial head implants in the treatment of unstable elbow fractures: a long-term review. J Trauma 2001;50:46-53.
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