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Selecting the diameter of a radial head implant: an assessment of local landmarks
J Shoulder Elbow Surg (2013) 22, 1395-1399
www.elsevier.com/locate/ymse
Selecting the diameter of a radial head implant: an assessment of local landmarks Bashar Alolabi, MDa,*, Alexis Studer, MDb, Alia Gray, MScc, Louis M. Ferreira, PhDc,d, Graham J.W. King, MDc,d, James A. Johnson, PhDc,d, George S. Athwal, MDc,d a
Orthopedic and Rheumatology Institute, Cleveland Clinic Foundation, Cleveland, OH, USA Alps Surgery Institute, Annecy, France c Department of Surgery, Western University, London, ON, Canada d Hand and Upper Limb Centre, St. Joseph’s Health Care, London, ON, Canada b
Introduction: Little information exists on radial head implant diameter sizing methods. When the native head is absent due to extensive comminution or previous excision, the lesser sigmoid notch may be a useful landmark for sizing. We evaluated the reliability of native radial head measurements, and the lesser sigmoid notch, as landmarks for radial head implant diameter sizing. Methods: We examined 27 fresh frozen ulnae and their corresponding radial heads. The maximum, minimum, and dish diameters of the radial heads were measured. A radial head implant diameter was selected based on the congruency of the trial implants with the radius of curvature of the lesser sigmoid notch. Intraobserver and interobserver reliability for all measurements and implant selection were assessed using intraclass correlation coefficients (ICC). Correlations between the native radial head measurements and the selected radial head implant diameter or the lesser sigmoid notch radius of curvature were assessed using the Pearson correlation coefficient (PCC). Results: Radial head diameter measurements demonstrated strong to excellent intraobserver (ICC 0.75) and interobserver reliability (ICC 0.82). The lesser sigmoid notch sizing method showed poor interobserver reliability (ICC ¼ 0.34). Only a moderate correlation was found between the native radial head and the lesser sigmoid notch (PCC 0.80) or the selected radial head implant size (PCC 0.59). Conclusion: Radial head diameter measurements showed excellent reliability, suggesting that the excised radial head, when available, should be used to select the implant diameter. The reliability of using the lesser sigmoid notch for sizing the diameter of radial head implants was only moderate, suggesting this is an unreliable landmark for implant diameter sizing. Level of evidence: Basic Science Study, Anatomy, Cadaver Dissection. Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Radial head replacement; lesser sigmoid notch; diameter; sizing
Investigational Review Board approval was not required for this study as per the University of Western Ontario Investigational Review Board. *Reprint requests: Bashar Alolabi, MD, FRCSC, St Joseph’s Health Care, Office of Dr G. Athwal, 268 Grosvenor St, Rm D0-205, London, ON N6A 4L6, Canada. E-mail address: [email protected] (B. Alolabi).
Radial head replacement is a commonly performed procedure for the treatment of comminuted radial head fractures.4,6-8 The selection of an implant that approximates the size of the native radial head is crucial to avoid pain, loss of motion, and post-traumatic arthritic changes.2,9,10,12 There are a number of published techniques to assist in the
1058-2746/$ - see front matter Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2013.04.005
1396 selection of an appropriately sized radial head height to prevent overlengthening of the radius or overstuffing of the radiocapitellar joint.1,3,5,11 There are no reports, however, with regards to choosing the correct diameter of radial head implants. Commonly, the excised fractured native radial head is assembled and used to choose a radial head implant diameter size that best matches the native head. Because the native radial head has an elliptical shape rather than a circular one, there is controversy about whether the maximum diameter (DMax), minimum diameter (DMin), or inner articular dish diameter (DDish) should be used for optimal sizing of the diameter of the radial head implant. No studies to date have evaluated the reliability of measuring these dimensions of the native radial head or have correlated other articular dimensions to the radial head diameter. Thus, we postulated that the radius of curvature of the lesser sigmoid notch could be a useful landmark, intraoperatively or using computed tomography (CT), when sizing of the radial head diameter is precluded by extensive comminution or revision surgery after a previous radial head excision. The purpose of this study was (1) to evaluate the reliability of measuring DMax, DMin, and DDish of the excised radial head, and (2) to determine if there is an anatomic or CT-based correlation between the radius of curvature of the lesser sigmoid notch and the radial head diameter and the efficacy of this landmark as a reliable method for radial head implant diameter sizing. We hypothesized that all measurements of the native radial head and the lesser sigmoid notch would be reliable landmarks for correct radial head implant diameter sizing.
Methods Reliability of measuring radial head dimensions Twenty-seven fresh frozen ulnae and their corresponding radial heads, from 18 male and 9 female donors, were thawed and denuded of all soft tissue. The DMax, DMin, and DDish were measured for each radial head using Digimatic CD-6 digital calipers (Mitutoyo, Tokyo, Japan; Fig. 1). Owing to the variable shape of the inner articular dish, the maximum diameter of the dish was used for DDish. Two investigators (B.A. and A.S.) performed the measurements, and these were repeated 3 to 5 weeks later. Intraobserver and interobserver reliability were calculated using the intraclass correlation coefficient (ICC).
Reliability of using the lesser sigmoid notch as an intraoperative landmark for radial head implant diameter sizing For each of the 27 proximal ulnae, 4 investigators independently selected a radial head implant diameter (Evolve; Wright Medical
B. Alolabi et al.
Figure 1 Image of a radial head demonstrates the maximum diameter (DMax), minimum diameter (DMin), and dish diameter (DDish).
Technology Inc, Arlington, TN, USA) based on its congruency with the radius of curvature of the lesser sigmoid notch, so that it forms a highly congruous fit (Fig. 2). Two investigators were upper-extremity fellows (B.A. and A.S.) and 2 were fellowship trained elbow surgeons (G.J.W.K. and G.S.A.). During the selection process, all investigators were blinded to the corresponding native radial heads. This selection was repeated 3 to 5 weeks later. Intraobserver and interobserver reliability was calculated using the intraclass correlation coefficient (ICC). Correlation between the native radial head diameter and the size of the selected radial head implant was performed using the Pearson correlation coefficient (PCC).
Reliability of using the lesser sigmoid notch as a CT-based landmark for radial head implant diameter sizing CT scans of acceptable quality were available for 23 (15 male and 8 female) the 27 denuded ulnae. The CTs were used to create 3-dimensional surface mesh and bone models using Mimics software (Materialise, Louvain, Belgium). Ten points were selected on the model surface within a transverse plane through the middle of the lesser sigmoid notch, orthogonal to the axis of rotation of the proximal radius (Fig. 3). A least-squares circle-fit determined the radius of curvature. Owing to the nonuniform shape of lesser sigmoid notch, 3 separate radii of curvature were calculated for the lesser sigmoid notch: 1 for the anterior half, 1 for the posterior half, and 1 for the total notch. Correlations between the 3 lesser sigmoid notch radii of curvature and the diameters (DMax, DMin, and DDish) of the corresponding native radial head, as measured by the digital calipers, were performed using PCC.
Diameter of a radial head implant
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Figure 2 Illustrations of a 3-dimensional model of the proximal ulna demonstrate how the radial head trial implant size was chosen based on congruency of the trial component with the radius of curvature (red dotted line) of the lesser sigmoid notch. The left figure demonstrates a radial head implant that is too small, whereas the larger radial head implant (right) is more congruent with the lesser sigmoid notch.
Figure 4 Graph demonstrates the correlation between the radial head maximum diameter and the radius of curvature of the lesser sigmoid notch, with a Pearson correlation coefficient of 0.72.
Figure 3 Illustration of a 3-dimensional model of the proximal ulna demonstrates how the radius of curvature of the lesser sigmoid notch was calculated by analyzing 10 markers (red dots) placed on its surface.
Results Reliability of measuring radial head dimensions The average DMax, DMin, and DDish of the native radial heads were 24.7 2.0, 23.1 1.9, and 17.3 1.4 mm, respectively. The intraobserver ICC was 0.99, 0.98, and 0.75 for the DMax, DMin and DDish, respectively. The maximum intraobserver difference in measurements was 0.6, 1.7, and 3.8 mm for the DMax, DMin, and DDish, respectively. Interobserver reliability testing demonstrated an ICC of 0.99, 1.00, and 0.82 for the DMax, DMin and DDish, respectively. The maximum difference in measurements between the 2 investigators was 0.7, 0.6, and 2.5 mm for the DMax, DMin, and DDish, respectively.
Reliability of using the lesser sigmoid notch as an intraoperative landmark for radial head implant diameter sizing The intraobserver reliability of selecting a radial head implant diameter size based on its anatomic congruency with the lesser sigmoid notch was good, with an ICC of 0.76. However, the interobserver reliability was poor, with an ICC of 0.34. Moreover, only moderate correlation was found between the average native radial head measurements and the selected radial head implant diameter, with a PCC of 0.56, 0.59, and 0.51 for the DMax, DMin, and DDish, respectively.
Reliability of using the lesser sigmoid notch as a radiographic landmark for radial head implant diameter sizing The average radius of curvature of the complete lesser sigmoid notch was 13.5 2.0 mm. The average radius of the anterior half of the lesser sigmoid notch was 18.4 7.4 mm and that of the posterior half was
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Figure 5 Graph demonstrates the correlation between the radial head minimum diameter and the radius of curvature of the lesser sigmoid notch, with a Pearson correlation coefficient of 0.74.
Figure 6 Graph demonstrates the correlation between the radial head dish diameter and the radius of curvature of the lesser sigmoid notch, with a Pearson correlation coefficient of 0.80.
10.2 2.1 mm. The correlation between the CT-based radius of curvature of the complete lesser sigmoid notch and the measurements of the native radial head showed a PCC of 0.72, 0.74, and 0.80, for the DMax, DMin, and DDish, respectively (Figs. 4-6). The correlation between the measurements of the native radial head and the radius of curvature of the anterior lesser sigmoid notch was much lower, with a PCC of 0.42, 0.41, and 0.44 for the DMax, DMin, and DDish, respectively. The correlation between the measurements of the native radial head and the radius of curvature of the posterior lesser sigmoid notch was also poor, with a PCC of 0.39, 0.35, and 0.35 for the DMax, DMin, and DDish, respectively.
Discussion When radial head replacement is performed, choosing an optimally sized implant that best approximates the native radial head is crucial. Studies have demonstrated that incorrect sizing can lead to suboptimal clinical results and long-term complications.2,9,10,12 Although there are a number of published techniques to guide the optimal choice of radial head implant height,1,3,5,11 no studies to date have addressed methods of selecting the diameter of a radial head implant. This study demonstrated that measuring the 3 common diameters (DMax, DMin, and
B. Alolabi et al. DDish) of the radial head had excellent intraobserver reliability, excellent interobserver reliability for DMax and DMin, and strong intraobserver reliability for DDish. This suggests that the excised native radial head, when available, should be used for radial head implant diameter sizing. However, the ICC for the DDish was lower than that for the DMax and DMin, suggesting that choosing the implant diameter size based on the DDish is not as reliable as choosing it based on the DMax or DMin. This is likely related to the variable shape of the inner articular dish as well as the difficulty in determining where the dish actually starts, especially when there is a gradual transition from the dish to the rim. We had postulated that the radius of curvature of the lesser sigmoid notch might correlate with the dimensions of the native radial head and, as such, would be a useful landmark for radial head implant diameter sizing when the radial head is unavailable. However, contrary to our hypothesis, this study demonstrated that using the lesser sigmoid notch to choose a radial head implant diameter size based on congruency of the notch and the trial implant has poor interobserver reliability. In addition, there was only a moderate correlation between the measurements of the native radial head and the selected trial implant diameter sizes based on the lesser sigmoid notch. This suggests that the curvature of the lesser sigmoid notch is an unreliable landmark for radial head implant diameter sizing. Furthermore, this study illustrated that there was a positive but only moderate correlation between the CT-based measurements of the radius of curvature of the lesser sigmoid notch and the measurements of the native radial head. In fact, there was a large difference between the radius of curvature of the anterior and posterior halves of the lesser sigmoid notch, implying that the lesser sigmoid notch is not uniform in shape. The correlation between the lesser sigmoid notch radius of curvature and the corresponding native radial head measurements was moderate to strong when the full notch was used to calculate the radius of curvature and weak when the anterior or posterior half was used. Hence, the lesser sigmoid notch should not be used as a landmark for radial head diameter sizing, neither CT-based nor intraoperatively. This is the first published report, to our knowledge, that evaluates the reliability of measuring the radial head dimensions as well as using the lesser sigmoid notch as landmarks for sizing the diameter of radial head implants. Presently, the best measurement parameter to select the diameter of a radial head implant is unknown. Some surgeons use the maximum outer diameter of the radial head, others the minimum outer diameter, and some use the maximum diameter of the articular dish. Future clinical and biomechanical studies are required to determine which parameter best recreates native elbow kinematics and optimizes contact mechanics and load transfer. The main limitation of this study is that the reliability testing of the native radial head measurements were
Diameter of a radial head implant performed on a nonfractured radial head, which would likely be different than measuring a fractured radial head that would require reassembly. In the future, a study designed to assess the reliability of measuring the dimensions of a fractured radial head would be useful. Other limitations include the small sample size and the unequal distribution of specimens from male and female donors. Moreover, CT measurements of the lesser sigmoid notch would only account for the osseous measurements and does not account for the cartilage. Finally, CT measurements of the lesser sigmoid notch radius of curvature were only performed by 1 observer with no evaluation of reliability.
Conclusion This study demonstrated that the native excised radial head, when available, should be used to choose the diameter of the radial head implant. When the radial head is too comminuted or unavailable, the lesser sigmoid notch is an unreliable intraoperative or CT-based landmark for radial head diameter sizing. Future studies should evaluate other local and nonlocal landmarks, such as the capitellum or the contralateral normal radial head, to assess their correlation with radial head measurements and their reliability in guiding correct radial head implant diameter sizing.
Disclaimer Funding for this project was provided through research grants by the Canadian Institute of Health Research and Physician’s Services Incorporated Foundation. Dr King is a consultant and receives royalties from Wright Medical Technology Inc. All other authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
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References 1. Athwal GS, Rouleau DM, MacDermid JC, King GJ. Contralateral elbow radiographs can reliably diagnose radial head implant overlengthening. J Bone Joint Surg Am 2011;93:1339-46. http://dx. doi.org/10.2106/JBJS.J.01244 2. Birkedal JP, Deal DN, Ruch DS. Loss of flexion after radial head replacement. J Shoulder Elbow Surg 2004;13:208-13. http://dx.doi. org/10.1016/j.jse.2003.11.007 3. Doornberg JN, Linzel DS, Zurakowski D, Ring D. Reference points for radial head prosthesis size. J Hand Surg Am 2006;31:53-7. http:// dx.doi.org/10.1016/j.jhsa.2005.06.012 4. Doornberg JN, Parisien R, van Duijn PJ, Ring D. Radial head arthroplasty with a modular metal spacer to treat acute traumatic elbow instability. J Bone Joint Surg Am 2007;89:1075-80. http://dx. doi.org/10.2106/JBJS.E.01340 5. Frank SG, Grewal R, Johnson J, Faber KJ, King GJ, Athwal GS. Determination of correct implant size in radial head arthroplasty to avoid overlengthening. J Bone Joint Surg Am 2009;91:1738-46. http:// dx.doi.org/10.2106/JBJS.H.01161 6. Grewal R, MacDermid JC, Faber KJ, Drosdowech DS, King GJ. Comminuted radial head fractures treated with a modular metallic radial head arthroplasty. Study of outcomes. J Bone Joint Surg Am 2006;88:2192-200. http://dx.doi.org/10.2106/JBJS.E.00962 7. Pugh DM, McKee MD. The ‘‘terrible triad’’ of the elbow. Tech Hand Up Extrem Surg 2002;6:21-9. http://dx.doi.org/10.1097/ 00130911-200203000-00005 8. Ring D, King G. Radial head arthroplasty with a modular metal spacer to treat acute traumatic elbow instability. Surgical technique. J Bone Joint Surg Am 2008;90(Suppl 2 Pt 1):63-73. http://dx.doi.org/10.2106/ JBJS.G.01248 9. Van Glabbeek F, van Riet RP, Baumfeld JA, Neale PG, O’Driscoll SW, Morrey BF, et al. The kinematic importance of radial neck length in radial head replacement. Med Eng Phys 2005;27:336-42. http://dx.doi. org/10.1016/j.medengphy.2004.04.011 10. Van Glabbeek F, Van Riet RP, Baumfeld JA, Neale PG, O’Driscoll SW, Morrey BF, et al. Detrimental effects of overstuffing or understuffing with a radial head replacement in the medial collateral-ligament deficient elbow. J Bone Joint Surg Am 2004;86: 2629-35. 11. Van Riet RP, van Glabbeek F, de Weerdt W, Oemar J, Bortier H. Validation of the lesser sigmoid notch of the ulna as a reference point for accurate placement of a prosthesis for the head of the radius: a cadaver study. J Bone Joint Surg Br 2007;89:413-6. http://dx.doi.org/ 10.1302/0301-620X.89B3.18099 12. Van Riet RP, Van Glabbeek F, Verborgt O, Gielen J. Capitellar erosion caused by a metal radial head prosthesis. A case report. J Bone Joint Surg Am 2004;86:1061-4.
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Selecting the diameter of a radial head implant: an assessment of local landmarks Bashar Alolabi, MDa,*, Alexis Studer, MDb, Alia Gray, MScc, Louis M. Ferreira, PhDc,d, Graham J.W. King, MDc,d, James A. Johnson, PhDc,d, George S. Athwal, MDc,d a
Orthopedic and Rheumatology Institute, Cleveland Clinic Foundation, Cleveland, OH, USA Alps Surgery Institute, Annecy, France c Department of Surgery, Western University, London, ON, Canada d Hand and Upper Limb Centre, St. Joseph’s Health Care, London, ON, Canada b
Introduction: Little information exists on radial head implant diameter sizing methods. When the native head is absent due to extensive comminution or previous excision, the lesser sigmoid notch may be a useful landmark for sizing. We evaluated the reliability of native radial head measurements, and the lesser sigmoid notch, as landmarks for radial head implant diameter sizing. Methods: We examined 27 fresh frozen ulnae and their corresponding radial heads. The maximum, minimum, and dish diameters of the radial heads were measured. A radial head implant diameter was selected based on the congruency of the trial implants with the radius of curvature of the lesser sigmoid notch. Intraobserver and interobserver reliability for all measurements and implant selection were assessed using intraclass correlation coefficients (ICC). Correlations between the native radial head measurements and the selected radial head implant diameter or the lesser sigmoid notch radius of curvature were assessed using the Pearson correlation coefficient (PCC). Results: Radial head diameter measurements demonstrated strong to excellent intraobserver (ICC 0.75) and interobserver reliability (ICC 0.82). The lesser sigmoid notch sizing method showed poor interobserver reliability (ICC ¼ 0.34). Only a moderate correlation was found between the native radial head and the lesser sigmoid notch (PCC 0.80) or the selected radial head implant size (PCC 0.59). Conclusion: Radial head diameter measurements showed excellent reliability, suggesting that the excised radial head, when available, should be used to select the implant diameter. The reliability of using the lesser sigmoid notch for sizing the diameter of radial head implants was only moderate, suggesting this is an unreliable landmark for implant diameter sizing. Level of evidence: Basic Science Study, Anatomy, Cadaver Dissection. Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Radial head replacement; lesser sigmoid notch; diameter; sizing
Investigational Review Board approval was not required for this study as per the University of Western Ontario Investigational Review Board. *Reprint requests: Bashar Alolabi, MD, FRCSC, St Joseph’s Health Care, Office of Dr G. Athwal, 268 Grosvenor St, Rm D0-205, London, ON N6A 4L6, Canada. E-mail address: [email protected] (B. Alolabi).
Radial head replacement is a commonly performed procedure for the treatment of comminuted radial head fractures.4,6-8 The selection of an implant that approximates the size of the native radial head is crucial to avoid pain, loss of motion, and post-traumatic arthritic changes.2,9,10,12 There are a number of published techniques to assist in the
1058-2746/$ - see front matter Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2013.04.005
1396 selection of an appropriately sized radial head height to prevent overlengthening of the radius or overstuffing of the radiocapitellar joint.1,3,5,11 There are no reports, however, with regards to choosing the correct diameter of radial head implants. Commonly, the excised fractured native radial head is assembled and used to choose a radial head implant diameter size that best matches the native head. Because the native radial head has an elliptical shape rather than a circular one, there is controversy about whether the maximum diameter (DMax), minimum diameter (DMin), or inner articular dish diameter (DDish) should be used for optimal sizing of the diameter of the radial head implant. No studies to date have evaluated the reliability of measuring these dimensions of the native radial head or have correlated other articular dimensions to the radial head diameter. Thus, we postulated that the radius of curvature of the lesser sigmoid notch could be a useful landmark, intraoperatively or using computed tomography (CT), when sizing of the radial head diameter is precluded by extensive comminution or revision surgery after a previous radial head excision. The purpose of this study was (1) to evaluate the reliability of measuring DMax, DMin, and DDish of the excised radial head, and (2) to determine if there is an anatomic or CT-based correlation between the radius of curvature of the lesser sigmoid notch and the radial head diameter and the efficacy of this landmark as a reliable method for radial head implant diameter sizing. We hypothesized that all measurements of the native radial head and the lesser sigmoid notch would be reliable landmarks for correct radial head implant diameter sizing.
Methods Reliability of measuring radial head dimensions Twenty-seven fresh frozen ulnae and their corresponding radial heads, from 18 male and 9 female donors, were thawed and denuded of all soft tissue. The DMax, DMin, and DDish were measured for each radial head using Digimatic CD-6 digital calipers (Mitutoyo, Tokyo, Japan; Fig. 1). Owing to the variable shape of the inner articular dish, the maximum diameter of the dish was used for DDish. Two investigators (B.A. and A.S.) performed the measurements, and these were repeated 3 to 5 weeks later. Intraobserver and interobserver reliability were calculated using the intraclass correlation coefficient (ICC).
Reliability of using the lesser sigmoid notch as an intraoperative landmark for radial head implant diameter sizing For each of the 27 proximal ulnae, 4 investigators independently selected a radial head implant diameter (Evolve; Wright Medical
B. Alolabi et al.
Figure 1 Image of a radial head demonstrates the maximum diameter (DMax), minimum diameter (DMin), and dish diameter (DDish).
Technology Inc, Arlington, TN, USA) based on its congruency with the radius of curvature of the lesser sigmoid notch, so that it forms a highly congruous fit (Fig. 2). Two investigators were upper-extremity fellows (B.A. and A.S.) and 2 were fellowship trained elbow surgeons (G.J.W.K. and G.S.A.). During the selection process, all investigators were blinded to the corresponding native radial heads. This selection was repeated 3 to 5 weeks later. Intraobserver and interobserver reliability was calculated using the intraclass correlation coefficient (ICC). Correlation between the native radial head diameter and the size of the selected radial head implant was performed using the Pearson correlation coefficient (PCC).
Reliability of using the lesser sigmoid notch as a CT-based landmark for radial head implant diameter sizing CT scans of acceptable quality were available for 23 (15 male and 8 female) the 27 denuded ulnae. The CTs were used to create 3-dimensional surface mesh and bone models using Mimics software (Materialise, Louvain, Belgium). Ten points were selected on the model surface within a transverse plane through the middle of the lesser sigmoid notch, orthogonal to the axis of rotation of the proximal radius (Fig. 3). A least-squares circle-fit determined the radius of curvature. Owing to the nonuniform shape of lesser sigmoid notch, 3 separate radii of curvature were calculated for the lesser sigmoid notch: 1 for the anterior half, 1 for the posterior half, and 1 for the total notch. Correlations between the 3 lesser sigmoid notch radii of curvature and the diameters (DMax, DMin, and DDish) of the corresponding native radial head, as measured by the digital calipers, were performed using PCC.
Diameter of a radial head implant
1397
Figure 2 Illustrations of a 3-dimensional model of the proximal ulna demonstrate how the radial head trial implant size was chosen based on congruency of the trial component with the radius of curvature (red dotted line) of the lesser sigmoid notch. The left figure demonstrates a radial head implant that is too small, whereas the larger radial head implant (right) is more congruent with the lesser sigmoid notch.
Figure 4 Graph demonstrates the correlation between the radial head maximum diameter and the radius of curvature of the lesser sigmoid notch, with a Pearson correlation coefficient of 0.72.
Figure 3 Illustration of a 3-dimensional model of the proximal ulna demonstrates how the radius of curvature of the lesser sigmoid notch was calculated by analyzing 10 markers (red dots) placed on its surface.
Results Reliability of measuring radial head dimensions The average DMax, DMin, and DDish of the native radial heads were 24.7 2.0, 23.1 1.9, and 17.3 1.4 mm, respectively. The intraobserver ICC was 0.99, 0.98, and 0.75 for the DMax, DMin and DDish, respectively. The maximum intraobserver difference in measurements was 0.6, 1.7, and 3.8 mm for the DMax, DMin, and DDish, respectively. Interobserver reliability testing demonstrated an ICC of 0.99, 1.00, and 0.82 for the DMax, DMin and DDish, respectively. The maximum difference in measurements between the 2 investigators was 0.7, 0.6, and 2.5 mm for the DMax, DMin, and DDish, respectively.
Reliability of using the lesser sigmoid notch as an intraoperative landmark for radial head implant diameter sizing The intraobserver reliability of selecting a radial head implant diameter size based on its anatomic congruency with the lesser sigmoid notch was good, with an ICC of 0.76. However, the interobserver reliability was poor, with an ICC of 0.34. Moreover, only moderate correlation was found between the average native radial head measurements and the selected radial head implant diameter, with a PCC of 0.56, 0.59, and 0.51 for the DMax, DMin, and DDish, respectively.
Reliability of using the lesser sigmoid notch as a radiographic landmark for radial head implant diameter sizing The average radius of curvature of the complete lesser sigmoid notch was 13.5 2.0 mm. The average radius of the anterior half of the lesser sigmoid notch was 18.4 7.4 mm and that of the posterior half was
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Figure 5 Graph demonstrates the correlation between the radial head minimum diameter and the radius of curvature of the lesser sigmoid notch, with a Pearson correlation coefficient of 0.74.
Figure 6 Graph demonstrates the correlation between the radial head dish diameter and the radius of curvature of the lesser sigmoid notch, with a Pearson correlation coefficient of 0.80.
10.2 2.1 mm. The correlation between the CT-based radius of curvature of the complete lesser sigmoid notch and the measurements of the native radial head showed a PCC of 0.72, 0.74, and 0.80, for the DMax, DMin, and DDish, respectively (Figs. 4-6). The correlation between the measurements of the native radial head and the radius of curvature of the anterior lesser sigmoid notch was much lower, with a PCC of 0.42, 0.41, and 0.44 for the DMax, DMin, and DDish, respectively. The correlation between the measurements of the native radial head and the radius of curvature of the posterior lesser sigmoid notch was also poor, with a PCC of 0.39, 0.35, and 0.35 for the DMax, DMin, and DDish, respectively.
Discussion When radial head replacement is performed, choosing an optimally sized implant that best approximates the native radial head is crucial. Studies have demonstrated that incorrect sizing can lead to suboptimal clinical results and long-term complications.2,9,10,12 Although there are a number of published techniques to guide the optimal choice of radial head implant height,1,3,5,11 no studies to date have addressed methods of selecting the diameter of a radial head implant. This study demonstrated that measuring the 3 common diameters (DMax, DMin, and
B. Alolabi et al. DDish) of the radial head had excellent intraobserver reliability, excellent interobserver reliability for DMax and DMin, and strong intraobserver reliability for DDish. This suggests that the excised native radial head, when available, should be used for radial head implant diameter sizing. However, the ICC for the DDish was lower than that for the DMax and DMin, suggesting that choosing the implant diameter size based on the DDish is not as reliable as choosing it based on the DMax or DMin. This is likely related to the variable shape of the inner articular dish as well as the difficulty in determining where the dish actually starts, especially when there is a gradual transition from the dish to the rim. We had postulated that the radius of curvature of the lesser sigmoid notch might correlate with the dimensions of the native radial head and, as such, would be a useful landmark for radial head implant diameter sizing when the radial head is unavailable. However, contrary to our hypothesis, this study demonstrated that using the lesser sigmoid notch to choose a radial head implant diameter size based on congruency of the notch and the trial implant has poor interobserver reliability. In addition, there was only a moderate correlation between the measurements of the native radial head and the selected trial implant diameter sizes based on the lesser sigmoid notch. This suggests that the curvature of the lesser sigmoid notch is an unreliable landmark for radial head implant diameter sizing. Furthermore, this study illustrated that there was a positive but only moderate correlation between the CT-based measurements of the radius of curvature of the lesser sigmoid notch and the measurements of the native radial head. In fact, there was a large difference between the radius of curvature of the anterior and posterior halves of the lesser sigmoid notch, implying that the lesser sigmoid notch is not uniform in shape. The correlation between the lesser sigmoid notch radius of curvature and the corresponding native radial head measurements was moderate to strong when the full notch was used to calculate the radius of curvature and weak when the anterior or posterior half was used. Hence, the lesser sigmoid notch should not be used as a landmark for radial head diameter sizing, neither CT-based nor intraoperatively. This is the first published report, to our knowledge, that evaluates the reliability of measuring the radial head dimensions as well as using the lesser sigmoid notch as landmarks for sizing the diameter of radial head implants. Presently, the best measurement parameter to select the diameter of a radial head implant is unknown. Some surgeons use the maximum outer diameter of the radial head, others the minimum outer diameter, and some use the maximum diameter of the articular dish. Future clinical and biomechanical studies are required to determine which parameter best recreates native elbow kinematics and optimizes contact mechanics and load transfer. The main limitation of this study is that the reliability testing of the native radial head measurements were
Diameter of a radial head implant performed on a nonfractured radial head, which would likely be different than measuring a fractured radial head that would require reassembly. In the future, a study designed to assess the reliability of measuring the dimensions of a fractured radial head would be useful. Other limitations include the small sample size and the unequal distribution of specimens from male and female donors. Moreover, CT measurements of the lesser sigmoid notch would only account for the osseous measurements and does not account for the cartilage. Finally, CT measurements of the lesser sigmoid notch radius of curvature were only performed by 1 observer with no evaluation of reliability.
Conclusion This study demonstrated that the native excised radial head, when available, should be used to choose the diameter of the radial head implant. When the radial head is too comminuted or unavailable, the lesser sigmoid notch is an unreliable intraoperative or CT-based landmark for radial head diameter sizing. Future studies should evaluate other local and nonlocal landmarks, such as the capitellum or the contralateral normal radial head, to assess their correlation with radial head measurements and their reliability in guiding correct radial head implant diameter sizing.
Disclaimer Funding for this project was provided through research grants by the Canadian Institute of Health Research and Physician’s Services Incorporated Foundation. Dr King is a consultant and receives royalties from Wright Medical Technology Inc. All other authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
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