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The radiocapitellar synovial fold: a lateral anatomic landmark for sizing radial head arthroplasty
ARTICLE IN PRESS J Shoulder Elbow Surg (2018) ■■, ■■–■■
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ORIGINAL ARTICLE
The radiocapitellar synovial fold: a lateral anatomic landmark for sizing radial head arthroplasty Alexander J. Lampley, MDa,*, Jacob W. Brubacher, MDb, Travis J. Dekker, MDa, Marc J. Richard, MDa, Grant E. Garrigues, MDa a
Department of Orthopaedic Surgery, Duke University Medical Center, Durham, NC, USA Department of Orthopedic Surgery, University of Kansas Medical Center, Kansas City, KS, USA
b
Background: Successful radial head arthroplasty relies on reproduction of anatomy. We hypothesized that the radiocapitellar synovial fold could serve as a reference point in radial head prosthesis sizing. Our study aimed to define the relationship between the synovial fold and the radial head in elbows with and without lateral ulnar collateral ligament (LUCL) injury. Materials and methods: We performed magnetic resonance imaging evaluation of 34 elbows to determine the normal relationship between the radiocapitellar synovial fold and the radial head. Next, we used cadaveric dissections to evaluate the anatomic relationships with the LUCL intact and disrupted, as well as in the setting of sizing with a radial head prosthesis. The fold–to–radial head distance (FRHD) was measured on all images and analyzed to determine the relationship of the synovial fold and radial head. Results: The FRHD in cadavers with an intact LUCL and native radial head measured an average of 1.5 mm proximal to the radial head. With the LUCL disrupted and a native radial head, the FRHD measured an average of 1.2 mm proximal to the radial head. The mean difference between the groups was 0.5 mm (P = .031), suggesting that the fold migrated distally in the cadavers with a disrupted LUCL. Conclusion: The radiocapitellar synovial fold may be a helpful landmark for radial head sizing. The synovial fold is always just proximal to the articular surface of the radial head. Using this information, the surgeon can prevent overlengthening as the implant should not be placed proximal to the fold. Level of evidence: Anatomy Study; Imaging and Cadaveric Dissection © 2018 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved. Keywords: Landmark; radial head; overstuffing; sizing; synovial fold; arthroplasty; elbow
This study (protocol ID Pro00071976) was conducted with the approval of the Duke Medicine Institutional Review Board for Clinical Investigations. *Reprint requests: Alexander J. Lampley, MD, Department of Orthopaedic Surgery, Duke University Medical Center, Box 3000, Durham, NC 27710, USA. E-mail address: [email protected] (A. Lampley).
Radial head fractures account for up to 5.4% of all fractures and can present a significant treatment challenge when comminution precludes anatomic reconstruction.6,11 While radial head resection has been shown to reduce pain, the radial head no longer acts as a secondary support to valgus stress, which may lead to valgus instability, proximal radial migration, ulnar-positive variance, altered elbow kinematics, and accelerated joint degeneration.20 Conversely, radial head arthroplasty provides pain relief and restores the native stability
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Figure 1 Sagittal magnetic resonance images demonstrating the similarities of the radiocapitellar synovial fold in the elbow (A, arrow) and the menisci of the knee (B, arrows). The most prominent aspect of the synovial fold curves around and sits right on the edge of the proximal radius (R) at the joint line similarly to the knee meniscus sitting between the tibial plateau and the distal femur (F). H, humerus; T, tibia.
and kinematics of the elbow but may be troubled by arthroplasty-related complications as discussed later.1 While the role of radial head replacement versus resection in the setting of intact elbow ligaments is debated, replacement is favored for unreconstructible, comminuted radial head fractures in the setting of concomitant elbow instability—where the radial head’s function as a secondary stabilizer becomes critical.18,24 Proper radial head sizing is critical to successful clinical outcomes as implanting a radial head prosthesis that is too large in length, known as “overstuffing” the radiocapitellar joint, can lead to loss of elbow motion, erosions of the capitellum, and early-onset arthritis secondary to alterations in elbow joint mechanics.2,21,23 There are few described intraoperative techniques to aid the surgeon in implanting a correctly sized prosthesis.10,13,22 The relationships between the native radial head and various anatomic structures, including the lateral edge of the coronoid, the lateral ulnohumeral joint space, and the lesser sigmoid notch, have been investigated as potential landmarks for accurate radial head sizing.10,13,22 While helpful in aiding the surgeon to implant an appropriately sized prosthesis, all of these anatomic landmarks exist medial to the radial head implant, and visualization after implantation of a trial or final prosthesis may be difficult. In addition, the radial head does not sit perpendicular to the forearm axis as the carrying angle of the elbow and curve of the proximal radius influence the orientation of the plane of the radial head articular surface with respect to the forearm axis. Thus, medially based landmarks may be insufficient to define the proper plane to restore the native radial head articular position. Lateral and posterior reference points in conjunction with the medial landmarks would allow the surgeon to more accurately define the plane to size and position the radial head prosthesis. In addition, visualization of a posterolateral anatomic structure would likely be easier to visualize through a laterally based surgical exposure.
During embryonic development, synovial folds or plicae are thought to be remnants of synovial membrane septa that are present during articular development.9,16 The radiocapitellar synovial fold is a posterolateral structure present in 86-98% of patients.12,15 It is similar to a knee meniscus in appearance as it has a semicircular shape with a triangular cross section that fits in the marginal contour of the articulation (Figs. 1 and 2).12 While the fold may play a protective role in forearm pronation and supination between the radius and capitellum, its mechanical importance is largely unknown.12 We hypothesized that the radiocapitellar synovial fold would be an anatomic structure of the joint capsule with a constant anatomic relationship to the radial head. We also hypothesized that this anatomic relationship between the synovial fold and radial head would not be changed by lateral ulnar collateral ligament (LUCL) injury and the synovial fold could serve as a supplementary reference point to aid in radial head prosthesis sizing.
Materials and methods Magnetic resonance imaging of radiocapitellar synovial fold To determine whether the radiocapitellar synovial fold is a structure with a constant anatomic relationship to the radial head, we retrospectively reviewed 34 magnetic resonance images (MRIs) without intra-articular contrast of the elbow. There were 14 female and 20 male patients with an average age of 37 years (range, 1772 years). The indications for the elbow MRIs included distal biceps tendon injuries in 7 patients, lateral epicondylitis in 6, intraarticular loose bodies in 2, medial epicondylitis of the elbow in 4, an osteochondral lesion of the capitellum in 1, a triceps tendon injury in 1, and partial medial ulnar collateral ligament injuries in 13. All patients were in the “anatomic position” with the elbow extended and the forearm supinated during the MRI scans.
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Figure 2 Coronal magnetic resonance images demonstrating the similarities of the radiocapitellar synovial fold in the elbow (A, arrow) and the menisci of the knee (B, arrows). The most prominent aspect of the synovial fold curves around and sits right on the edge of the proximal radius (R) at the joint line similarly to the knee meniscus sitting between the tibial plateau and the distal femur (F). H, humerus; T, tibia.
Figure 3 Measurement of the fold–to–radial head distance in the coronal plane (A) and sagittal plane (B), which is the perpendicular distance (black line) from the plane of the radial head (line a) to the most prominent aspect of the synovial fold. The fold–to–radial head distance (black line) measures 1.02 mm in the coronal plane and 0.66 mm in the sagittal plane. H, humerus; U, ulna; R, radius. By use of T2-weighted images, the radial synovial fold was identified on 30 of 34 MRIs (88%). The fold was measured on coronal and sagittal cuts. The fold–to–radial head distance (FRHD) was defined by the perpendicular distance from the tip of the fold to a plane connecting the apices of the subchondral surface of the 2 most proximal aspects of the radial head. The FRHD was measured in the coronal and sagittal planes (Fig. 3). Negative values indicate that the fold was distal to the axis of the radial head subchondral surface. All MRI measurements were performed using imaging software (Centricity Enterprise Web; General Electric Medical Systems, Fairfield, CT, USA). We evaluated interobserver reliability of the measurement technique with 3 observers independently performing all measurements. To evaluate intraobserver reliability, we then had the 3 observers repeat the measurements on the same 10 randomly selected MRIs 2 times with a 1-week interval between measurements.
Cadaveric dissection To confirm that the anatomic relationship between the synovial fold and radial head is not changed by LUCL injury, we used 9 freshfrozen cadaveric elbows that were disarticulated at the glenohumeral joint. The mean age of the specimens was 69.3 years (range, 5683 years), and there were 3 male and 6 female specimens. All elbows were examined and found to be free of elbow arthritis, congenital anomaly, gross oncologic process, or prior elbow surgery. We obtained 3 series of anteroposterior (AP) elbow fluoroscopic images with the forearm supinated at different stages of the study for each specimen. We made an extensor digitorum communis split approach7 in each specimen, making sure to keep the initial dissection anterior to the insertion of the LUCL on the lateral epicondyle and in the mid axis
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Figure 4 Cadaveric specimen after extensor split approach showing the radial head (R), distal humerus (H), and radiocapitellar synovial fold (arrow). The radiocapitellar synovial fold is a posterolateral structure.
A.J. Lampley et al.
Figure 6 Cadaveric specimen with simulated lateral ulnar collateral ligament avulsion by stripping the lateral humeral column. The radiocapitellar synovial fold is labeled with microclips (arrow). The radial head has been excised in preparation for radial head replacement. H, distal humerus; R, proximal radius; LS, lesser sigmoid notch.
Figure 5 Anteroposterior fluoroscopic image with microclips applied to the radiocapitellar synovial fold (arrow) and an intact lateral ulnar collateral ligament. H, humerus; U, ulna; R, radius. of the radius. The radiocapitellar synovial fold was present just proximal to the articular surface of the radial head in all specimens (Fig. 4). A microclip applier (Covidien, Dublin, Ireland) was used to label the fold with 2 clips. The first AP fluoroscopic image was taken with the native radial head in place with an intact LUCL and microclips applied to the synovial fold (Fig. 5). A scalpel was then used to strip the lateral humeral column and condyle of all attachments to simulate an avulsion of the lateral ligaments and the common extensor attachment (Fig. 6). We then obtained a second AP fluoroscopic image with the native radial head in place, a detached LUCL, and microclips applied to the synovial fold (Fig. 7). The radial head was resected, and an anatomic radial head system (Acumed, Hillsboro, OR, USA) was used to complete the steps of radial head replacement. The final implant was selected to match the measured resected radial head to restore proper radial head height (Fig. 8). The last AP fluoroscopic image was obtained with the radial head prosthesis implanted, a detached LUCL, and microclips applied
Figure 7 Anteroposterior fluoroscopic image with microclips applied to the radiocapitellar synovial fold (arrow) and a simulated lateral ulnar collateral ligament avulsion. H, humerus; R, radius; U, ulna.
to the synovial fold (Fig. 9). The 3 senior authors (J.W.B., G.E.G., and M.J.R.) agreed that each final implant was appropriately sized before the final implant was placed, using the established clinical landmarks of the lateral edge of the coronoid, the lateral ulnohumeral joint space on the AP radiograph, and the lesser sigmoid notch.10,13,22 Three authors then performed measurements on each set of radiographs to evaluate the relationship between the synovial fold and radial head. The FRHD was defined by the perpendicular distance from the 2 clips in the synovial fold and a line drawn tangentially to the subchondral bone of the radial head articular surface (Fig. 10). Negative values indicate that the synovial fold was distal to the axis of the radial head subchondral surface.
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Figure 8 Cadaveric specimen with radial head replacement (R). The radiocapitellar synovial fold remains labeled with microclips (arrow). Again, the synovial fold is a posterolateral anatomic structure. H, distal humerus. Figure 10 Measurement of the fold–to–radial head distance, which is the perpendicular distance (white barbell-shaped line) from the plane of the radial head (line a) to the microclips. The fold–to– radial head distance (white barbell-shaped line) measures 1.18 mm. This measurement was performed on each anteroposterior fluoroscopic image. H, humerus; U, ulna; R, radius.
Figure 9 Anteroposterior fluoroscopic image after radial head replacement with microclips applied to the radiocapitellar synovial fold (arrow) and a simulated lateral ulnar collateral ligament avulsion. H, humerus; R, radius; U, ulna. All radiograph measurements were performed using imaging software (Centricity Enterprise Web). We evaluated interobserver and intraobserver reliability of the measurement technique with 3 observers independently performing all 9 FRHD measurements and then repeating measurements on 4 elbows 1 week after the initial measurements were done.
Statistical methods Continuous data are presented using the mean, standard deviation (SD), and range, while categorical data are summarized using counts and percentages. Measurements on each set of radiographs were taken to compare the synovial fold and radial head, including the FRHD, within 3 specific groups (LUCL intact–native radial head, LUCL disrupted–native radial head, and LUCL disrupted–prosthetic radial
head). Comparisons between groups were done using the t test, and the average measurement across observers within each group was used to derive the summary statistics used for comparison. P < .05 was considered statistically significant, and SAS software (version 9.4; SAS Institute, Cary, NC, USA) was used for all analyses. The intraclass correlation coefficient (ICC) was used to measure the level of both intraobserver reliability and interobserver reliability for FRHD measurements within the 3 groups previously discussed. The ICC assessed the consistency of measurements made by the 3 observers measuring the same quantity for interobserver reliability and repeated measurements on 4 elbows for intraobserver reliability. The ICC is given along with the 95% confidence interval (CI) across all FRHD measures and then by subgroup. Correlations of less than 0.60 were considered poor to fair; between 0.60 and 0.74, good; and above 0.75, excellent.5
Results While the synovial fold was present on 88% of the reviewed MRIs, the fold was present in all cadaveric specimens and reliably identified the radiocapitellar joint—immediately adjacent and proximal to the radial head cartilage. When traced along the capsule, the semicircular fold also marked the proximal edge of the lesser sigmoid notch articular cartilage. On MRI, the average FRHD was 0.9 mm (SD, 0.5 mm; range, 0.1-2.8 mm) in the coronal plane and 0.8 mm (SD, 0.4 mm; range 0.3-2.9 mm) in the sagittal plane. All FRHD measurements were positive, indicating that the synovial fold was always proximal to the plane defined by the subchondral surface of the radial head. In cadaveric specimens, the FRHD in cadavers with an intact LUCL and native radial head measured an average of
ARTICLE IN PRESS 6 1.5 ± 0.4 mm (range, 0.9-2.3 mm). With the LUCL disrupted and a native radial head, the FRHD measured an average of 1.2 ± 0.6 mm (range, 0.4-2.3 mm). The FRHD with the LUCL disrupted and the radial head prosthesis in place averaged 0.5 ± 0.8 mm (range, 0-2.7 mm). A statistically significant difference was found when the FRHD in cadavers with an intact LUCL and native radial head was compared with the FRHD in cadavers with a disrupted LUCL and a native head. The mean difference between the groups was 0.5 ± 0.4 mm (95% CI, 0.1-0.8 mm; P = .031), which suggests that the radiocapitellar fold migrated distally in the cadavers with a disrupted LUCL.
Measurement reliability We found excellent interobserver reliability among the 3 observers for all measurements, with an ICC value of 0.91 (95% CI, 0.83-0.95) for the FRHD across all cadavers. Excellent reliability was also found for the FRHD in the cadavers with an intact LUCL and native radial head, with an ICC value of 0.79 (95% CI, 0.49-0.93); in the cadavers with a disrupted LUCL and native radial head, with an ICC of 0.82 (95% CI, 0.50-0.95); and in the cadavers with a disrupted LUCL and replacement radial head, with an ICC of 0.94 (95% CI, 0.83-0.98). The average difference in MRI measurements between observers for the FRHD in the coronal and sagittal planes was 0.2 mm. The average difference in fluoroscopic measurements between observers in the cadaveric FRHD for all specimens was 0.3 mm. With regard to intraobserver reliability, we again found excellent intraobserver reliability among the 3 observers for all measurements, with an ICC value of 0.92 (95% CI, 0.840.96) for the FRHD across all cadavers. In the cadavers with an intact LUCL and native radial head, we found an ICC value of 0.82 (95% CI, 0.50-0.95) for the FRHD; in the cadavers with a disrupted LUCL and native radial head, 0.90 (95% CI, 0.63-0.98); and in the cadavers with a disrupted LUCL and replacement radial head, 0.80 (95% CI, 0.45-0.94).
Discussion Sizing of radial head arthroplasty continues to be a clinical challenge and can lead to poor results when complicated by overlengthening.2,21,23 Several methods have been described to help with sizing the radial head prosthesis. Doornberg et al10 studied the relationship between the radial head and the proximal aspect of the lesser sigmoid notch after reviewing 17 computed tomography scans. While the radial head was on average 0.9 mm proximal to the proximal aspect of the lesser sigmoid notch, there was significant variability among the patients, with some patients’ radial heads sitting proximal to the lesser sigmoid notch and some radial heads sitting distal to the lesser sigmoid notch. Likewise, van Riet et al22 described the relationship between the radial head and the lesser sigmoid notch in cadaveric specimens. After resecting the radial
A.J. Lampley et al. head from the cadaveric specimens, they found that the resected radial head length was roughly equivalent to the distance from the radial neck cut to the proximal edge of the lesser sigmoid notch. Therefore, in the operating room, the measured length from the proximal radial neck to the proximal lesser sigmoid notch can be used to determine the size of the radial head. In another cadaveric experiment, Frank et al13 showed that the lateral ulnohumeral joint surfaces articulated without a gap when a correctly sized radial head prosthesis was implanted. When the radial head prosthesis was overstuffed by 2 mm or more, visible gapping was seen at the lateral ulnohumeral joint. However, it is important to note that Rowland et al19 reviewed 50 normal AP elbow radiographs and found that the lateral ulnohumeral joint space was wider than the medial ulnohumeral joint space on 94% of the radiographs. In addition, the lateral ulnohumeral joint space was 1 mm or greater in width than the medial ulnohumeral joint space in 32% of the normal elbows. Thus, the lateral ulnohumeral joint space is normally wider than the medial joint space, and this sign may not be a reliable marker of overlengthening the radial head prosthesis during radial head replacement. While these methods are valuable in implanting an appropriately sized radial head, they are difficult to apply as accurate intraoperative measurement is challenging, and it is difficult to visualize these medial landmarks once the trial or final prosthesis is implanted. Using MRIs, we demonstrated that the radiocapitellar synovial fold is a structure with a constant anatomic relationship to the radial head. Our measurements indicate that the plane defined by the proximal subchondral bone of the most proximal aspect of the radial head sits slightly less than 1 mm distal to the most prominent aspect of the radiocapitellar synovial fold in both the coronal and sagittal planes. Although there was a small degree of variability in this measurement between patients, the native radial head was always distal to the synovial fold. Similarly, this anatomic relationship was confirmed in our cadaveric specimens as the fold was present in all specimens and averaged 1.5 mm from the subchondral surface of the radial head on radiographs. However, it is important to note that the FRHD was measured from the subchondral bone on the radiographs, and the majority of this distance was likely made up of the thickness of the radial head articular cartilage. After the radial head was replaced, the FRHD was 0.5 mm. This decrease in distance to the fold represents the replacement of the radial head articular surface with the metal implant, which—unlike articular cartilage—can be measured on radiographs. Yeung et al25 showed cartilage thickness around the rim of the radial head to have regional variability, with cartilage thickness ranging from 0.8 to 1.7 mm. Applying the range of cartilage thickness to our data range, we can predict that the synovial fold sits on average 0.10.5 mm from the subchondral bone, which agrees well with our FRHD after radial head replacement. Furthermore, we reported an average FRHD of 0.9 mm after reviewing 30 MRIs of normal elbows, which also agrees with our data.
ARTICLE IN PRESS Novel radial head replacement reference point To determine whether the relationship between the radiocapitellar synovial fold and the radial head was affected by injury, we simulated significant soft-tissue damage to the lateral aspect of the humerus by stripping off all soft-tissue attachments on the lateral epicondyle and lateral ridge of the humerus to include the LUCL and the common extensor origin. This type of soft-tissue injury is frequently associated with comminuted radial head fractures,4 and this type of cadaveric model has been used in prior studies of this injury pattern.3,8,14,17 We found a statistically significant distal migration of the synovial fold by an average of 0.5 mm. While a simulated lateral soft-tissue injury did change the FRHD, a difference of less than 0.5 mm is likely not clinically significant as this small amount of change would have little impact on the sizing of the radial head arthroplasty. In fact, Van Glabbeek et al21 demonstrated that changes were seen in elbow kinematics only by overlengthening the radius by 2.5 mm or greater. The limitations of our study include a relatively small number of specimens. This cadaveric study attempts to re-create an LUCL injury with soft-tissue stripping, and it is unknown whether this accurately simulates the clinical scenario of terribletriad fracture-dislocation. However, this technique has been used in other cadaveric studies to simulate LUCL injury.3,8,14,17 Although our experience has shown the synovial fold to be visible and useful in the setting of trauma, further clinical studies will be helpful to define the usefulness of the synovial fold in radial head sizing in these clinical situations. We evaluated the use of the radiocapitellar synovial fold as a landmark to clinically help find the appropriate radial head replacement length. Both cadaveric and MRI evaluations confirmed that the radiocapitellar synovial fold is an anatomic structure with a constant anatomic relationship to the radial head. We also demonstrated that this anatomic relationship between the synovial fold and radial head is not significantly changed by a simulated LUCL injury in a cadaveric model. The proximity of the fold to the radial head confirms the potential benefit as a guide when sizing the radial head arthroplasty length.
Conclusion Our study is meant to supplement the existing sizing references. We believe the radiocapitellar synovial fold may be of significant utility as a radial head sizing landmark given that this is a posterolateral structure—both making it easily visualized while performing the lateral approach for the arthroplasty and allowing the proper plane for the radial head articular surface to be determined in conjunction with the medial landmarks. This study shows that the radiocapitellar synovial fold is on average 0.8-1.5 mm from the subchondral bone of the radial head and should always be proximal to the articular surface of the radial head. Using this information, the surgeon can prevent overstuffing as the implant should not be placed proximal to the synovial fold.
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Acknowledgment The authors thank Dr. Matthew Ramsey for providing the inspiration for this investigation.
Disclaimer This cadaveric study was completed with an educational grant in the amount of $19,000 from Acumed (Hillsboro, OR, USA). The outside source of funds was not involved in data collection, data analysis, or preparation or editing of the manuscript. The outside source of funds was used to fund the cadaveric specimens and implants used in the study. Marc J. Richard is a paid consultant for Acumed. All the 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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ORIGINAL ARTICLE
The radiocapitellar synovial fold: a lateral anatomic landmark for sizing radial head arthroplasty Alexander J. Lampley, MDa,*, Jacob W. Brubacher, MDb, Travis J. Dekker, MDa, Marc J. Richard, MDa, Grant E. Garrigues, MDa a
Department of Orthopaedic Surgery, Duke University Medical Center, Durham, NC, USA Department of Orthopedic Surgery, University of Kansas Medical Center, Kansas City, KS, USA
b
Background: Successful radial head arthroplasty relies on reproduction of anatomy. We hypothesized that the radiocapitellar synovial fold could serve as a reference point in radial head prosthesis sizing. Our study aimed to define the relationship between the synovial fold and the radial head in elbows with and without lateral ulnar collateral ligament (LUCL) injury. Materials and methods: We performed magnetic resonance imaging evaluation of 34 elbows to determine the normal relationship between the radiocapitellar synovial fold and the radial head. Next, we used cadaveric dissections to evaluate the anatomic relationships with the LUCL intact and disrupted, as well as in the setting of sizing with a radial head prosthesis. The fold–to–radial head distance (FRHD) was measured on all images and analyzed to determine the relationship of the synovial fold and radial head. Results: The FRHD in cadavers with an intact LUCL and native radial head measured an average of 1.5 mm proximal to the radial head. With the LUCL disrupted and a native radial head, the FRHD measured an average of 1.2 mm proximal to the radial head. The mean difference between the groups was 0.5 mm (P = .031), suggesting that the fold migrated distally in the cadavers with a disrupted LUCL. Conclusion: The radiocapitellar synovial fold may be a helpful landmark for radial head sizing. The synovial fold is always just proximal to the articular surface of the radial head. Using this information, the surgeon can prevent overlengthening as the implant should not be placed proximal to the fold. Level of evidence: Anatomy Study; Imaging and Cadaveric Dissection © 2018 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved. Keywords: Landmark; radial head; overstuffing; sizing; synovial fold; arthroplasty; elbow
This study (protocol ID Pro00071976) was conducted with the approval of the Duke Medicine Institutional Review Board for Clinical Investigations. *Reprint requests: Alexander J. Lampley, MD, Department of Orthopaedic Surgery, Duke University Medical Center, Box 3000, Durham, NC 27710, USA. E-mail address: [email protected] (A. Lampley).
Radial head fractures account for up to 5.4% of all fractures and can present a significant treatment challenge when comminution precludes anatomic reconstruction.6,11 While radial head resection has been shown to reduce pain, the radial head no longer acts as a secondary support to valgus stress, which may lead to valgus instability, proximal radial migration, ulnar-positive variance, altered elbow kinematics, and accelerated joint degeneration.20 Conversely, radial head arthroplasty provides pain relief and restores the native stability
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Figure 1 Sagittal magnetic resonance images demonstrating the similarities of the radiocapitellar synovial fold in the elbow (A, arrow) and the menisci of the knee (B, arrows). The most prominent aspect of the synovial fold curves around and sits right on the edge of the proximal radius (R) at the joint line similarly to the knee meniscus sitting between the tibial plateau and the distal femur (F). H, humerus; T, tibia.
and kinematics of the elbow but may be troubled by arthroplasty-related complications as discussed later.1 While the role of radial head replacement versus resection in the setting of intact elbow ligaments is debated, replacement is favored for unreconstructible, comminuted radial head fractures in the setting of concomitant elbow instability—where the radial head’s function as a secondary stabilizer becomes critical.18,24 Proper radial head sizing is critical to successful clinical outcomes as implanting a radial head prosthesis that is too large in length, known as “overstuffing” the radiocapitellar joint, can lead to loss of elbow motion, erosions of the capitellum, and early-onset arthritis secondary to alterations in elbow joint mechanics.2,21,23 There are few described intraoperative techniques to aid the surgeon in implanting a correctly sized prosthesis.10,13,22 The relationships between the native radial head and various anatomic structures, including the lateral edge of the coronoid, the lateral ulnohumeral joint space, and the lesser sigmoid notch, have been investigated as potential landmarks for accurate radial head sizing.10,13,22 While helpful in aiding the surgeon to implant an appropriately sized prosthesis, all of these anatomic landmarks exist medial to the radial head implant, and visualization after implantation of a trial or final prosthesis may be difficult. In addition, the radial head does not sit perpendicular to the forearm axis as the carrying angle of the elbow and curve of the proximal radius influence the orientation of the plane of the radial head articular surface with respect to the forearm axis. Thus, medially based landmarks may be insufficient to define the proper plane to restore the native radial head articular position. Lateral and posterior reference points in conjunction with the medial landmarks would allow the surgeon to more accurately define the plane to size and position the radial head prosthesis. In addition, visualization of a posterolateral anatomic structure would likely be easier to visualize through a laterally based surgical exposure.
During embryonic development, synovial folds or plicae are thought to be remnants of synovial membrane septa that are present during articular development.9,16 The radiocapitellar synovial fold is a posterolateral structure present in 86-98% of patients.12,15 It is similar to a knee meniscus in appearance as it has a semicircular shape with a triangular cross section that fits in the marginal contour of the articulation (Figs. 1 and 2).12 While the fold may play a protective role in forearm pronation and supination between the radius and capitellum, its mechanical importance is largely unknown.12 We hypothesized that the radiocapitellar synovial fold would be an anatomic structure of the joint capsule with a constant anatomic relationship to the radial head. We also hypothesized that this anatomic relationship between the synovial fold and radial head would not be changed by lateral ulnar collateral ligament (LUCL) injury and the synovial fold could serve as a supplementary reference point to aid in radial head prosthesis sizing.
Materials and methods Magnetic resonance imaging of radiocapitellar synovial fold To determine whether the radiocapitellar synovial fold is a structure with a constant anatomic relationship to the radial head, we retrospectively reviewed 34 magnetic resonance images (MRIs) without intra-articular contrast of the elbow. There were 14 female and 20 male patients with an average age of 37 years (range, 1772 years). The indications for the elbow MRIs included distal biceps tendon injuries in 7 patients, lateral epicondylitis in 6, intraarticular loose bodies in 2, medial epicondylitis of the elbow in 4, an osteochondral lesion of the capitellum in 1, a triceps tendon injury in 1, and partial medial ulnar collateral ligament injuries in 13. All patients were in the “anatomic position” with the elbow extended and the forearm supinated during the MRI scans.
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Figure 2 Coronal magnetic resonance images demonstrating the similarities of the radiocapitellar synovial fold in the elbow (A, arrow) and the menisci of the knee (B, arrows). The most prominent aspect of the synovial fold curves around and sits right on the edge of the proximal radius (R) at the joint line similarly to the knee meniscus sitting between the tibial plateau and the distal femur (F). H, humerus; T, tibia.
Figure 3 Measurement of the fold–to–radial head distance in the coronal plane (A) and sagittal plane (B), which is the perpendicular distance (black line) from the plane of the radial head (line a) to the most prominent aspect of the synovial fold. The fold–to–radial head distance (black line) measures 1.02 mm in the coronal plane and 0.66 mm in the sagittal plane. H, humerus; U, ulna; R, radius. By use of T2-weighted images, the radial synovial fold was identified on 30 of 34 MRIs (88%). The fold was measured on coronal and sagittal cuts. The fold–to–radial head distance (FRHD) was defined by the perpendicular distance from the tip of the fold to a plane connecting the apices of the subchondral surface of the 2 most proximal aspects of the radial head. The FRHD was measured in the coronal and sagittal planes (Fig. 3). Negative values indicate that the fold was distal to the axis of the radial head subchondral surface. All MRI measurements were performed using imaging software (Centricity Enterprise Web; General Electric Medical Systems, Fairfield, CT, USA). We evaluated interobserver reliability of the measurement technique with 3 observers independently performing all measurements. To evaluate intraobserver reliability, we then had the 3 observers repeat the measurements on the same 10 randomly selected MRIs 2 times with a 1-week interval between measurements.
Cadaveric dissection To confirm that the anatomic relationship between the synovial fold and radial head is not changed by LUCL injury, we used 9 freshfrozen cadaveric elbows that were disarticulated at the glenohumeral joint. The mean age of the specimens was 69.3 years (range, 5683 years), and there were 3 male and 6 female specimens. All elbows were examined and found to be free of elbow arthritis, congenital anomaly, gross oncologic process, or prior elbow surgery. We obtained 3 series of anteroposterior (AP) elbow fluoroscopic images with the forearm supinated at different stages of the study for each specimen. We made an extensor digitorum communis split approach7 in each specimen, making sure to keep the initial dissection anterior to the insertion of the LUCL on the lateral epicondyle and in the mid axis
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Figure 4 Cadaveric specimen after extensor split approach showing the radial head (R), distal humerus (H), and radiocapitellar synovial fold (arrow). The radiocapitellar synovial fold is a posterolateral structure.
A.J. Lampley et al.
Figure 6 Cadaveric specimen with simulated lateral ulnar collateral ligament avulsion by stripping the lateral humeral column. The radiocapitellar synovial fold is labeled with microclips (arrow). The radial head has been excised in preparation for radial head replacement. H, distal humerus; R, proximal radius; LS, lesser sigmoid notch.
Figure 5 Anteroposterior fluoroscopic image with microclips applied to the radiocapitellar synovial fold (arrow) and an intact lateral ulnar collateral ligament. H, humerus; U, ulna; R, radius. of the radius. The radiocapitellar synovial fold was present just proximal to the articular surface of the radial head in all specimens (Fig. 4). A microclip applier (Covidien, Dublin, Ireland) was used to label the fold with 2 clips. The first AP fluoroscopic image was taken with the native radial head in place with an intact LUCL and microclips applied to the synovial fold (Fig. 5). A scalpel was then used to strip the lateral humeral column and condyle of all attachments to simulate an avulsion of the lateral ligaments and the common extensor attachment (Fig. 6). We then obtained a second AP fluoroscopic image with the native radial head in place, a detached LUCL, and microclips applied to the synovial fold (Fig. 7). The radial head was resected, and an anatomic radial head system (Acumed, Hillsboro, OR, USA) was used to complete the steps of radial head replacement. The final implant was selected to match the measured resected radial head to restore proper radial head height (Fig. 8). The last AP fluoroscopic image was obtained with the radial head prosthesis implanted, a detached LUCL, and microclips applied
Figure 7 Anteroposterior fluoroscopic image with microclips applied to the radiocapitellar synovial fold (arrow) and a simulated lateral ulnar collateral ligament avulsion. H, humerus; R, radius; U, ulna.
to the synovial fold (Fig. 9). The 3 senior authors (J.W.B., G.E.G., and M.J.R.) agreed that each final implant was appropriately sized before the final implant was placed, using the established clinical landmarks of the lateral edge of the coronoid, the lateral ulnohumeral joint space on the AP radiograph, and the lesser sigmoid notch.10,13,22 Three authors then performed measurements on each set of radiographs to evaluate the relationship between the synovial fold and radial head. The FRHD was defined by the perpendicular distance from the 2 clips in the synovial fold and a line drawn tangentially to the subchondral bone of the radial head articular surface (Fig. 10). Negative values indicate that the synovial fold was distal to the axis of the radial head subchondral surface.
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Figure 8 Cadaveric specimen with radial head replacement (R). The radiocapitellar synovial fold remains labeled with microclips (arrow). Again, the synovial fold is a posterolateral anatomic structure. H, distal humerus. Figure 10 Measurement of the fold–to–radial head distance, which is the perpendicular distance (white barbell-shaped line) from the plane of the radial head (line a) to the microclips. The fold–to– radial head distance (white barbell-shaped line) measures 1.18 mm. This measurement was performed on each anteroposterior fluoroscopic image. H, humerus; U, ulna; R, radius.
Figure 9 Anteroposterior fluoroscopic image after radial head replacement with microclips applied to the radiocapitellar synovial fold (arrow) and a simulated lateral ulnar collateral ligament avulsion. H, humerus; R, radius; U, ulna. All radiograph measurements were performed using imaging software (Centricity Enterprise Web). We evaluated interobserver and intraobserver reliability of the measurement technique with 3 observers independently performing all 9 FRHD measurements and then repeating measurements on 4 elbows 1 week after the initial measurements were done.
Statistical methods Continuous data are presented using the mean, standard deviation (SD), and range, while categorical data are summarized using counts and percentages. Measurements on each set of radiographs were taken to compare the synovial fold and radial head, including the FRHD, within 3 specific groups (LUCL intact–native radial head, LUCL disrupted–native radial head, and LUCL disrupted–prosthetic radial
head). Comparisons between groups were done using the t test, and the average measurement across observers within each group was used to derive the summary statistics used for comparison. P < .05 was considered statistically significant, and SAS software (version 9.4; SAS Institute, Cary, NC, USA) was used for all analyses. The intraclass correlation coefficient (ICC) was used to measure the level of both intraobserver reliability and interobserver reliability for FRHD measurements within the 3 groups previously discussed. The ICC assessed the consistency of measurements made by the 3 observers measuring the same quantity for interobserver reliability and repeated measurements on 4 elbows for intraobserver reliability. The ICC is given along with the 95% confidence interval (CI) across all FRHD measures and then by subgroup. Correlations of less than 0.60 were considered poor to fair; between 0.60 and 0.74, good; and above 0.75, excellent.5
Results While the synovial fold was present on 88% of the reviewed MRIs, the fold was present in all cadaveric specimens and reliably identified the radiocapitellar joint—immediately adjacent and proximal to the radial head cartilage. When traced along the capsule, the semicircular fold also marked the proximal edge of the lesser sigmoid notch articular cartilage. On MRI, the average FRHD was 0.9 mm (SD, 0.5 mm; range, 0.1-2.8 mm) in the coronal plane and 0.8 mm (SD, 0.4 mm; range 0.3-2.9 mm) in the sagittal plane. All FRHD measurements were positive, indicating that the synovial fold was always proximal to the plane defined by the subchondral surface of the radial head. In cadaveric specimens, the FRHD in cadavers with an intact LUCL and native radial head measured an average of
ARTICLE IN PRESS 6 1.5 ± 0.4 mm (range, 0.9-2.3 mm). With the LUCL disrupted and a native radial head, the FRHD measured an average of 1.2 ± 0.6 mm (range, 0.4-2.3 mm). The FRHD with the LUCL disrupted and the radial head prosthesis in place averaged 0.5 ± 0.8 mm (range, 0-2.7 mm). A statistically significant difference was found when the FRHD in cadavers with an intact LUCL and native radial head was compared with the FRHD in cadavers with a disrupted LUCL and a native head. The mean difference between the groups was 0.5 ± 0.4 mm (95% CI, 0.1-0.8 mm; P = .031), which suggests that the radiocapitellar fold migrated distally in the cadavers with a disrupted LUCL.
Measurement reliability We found excellent interobserver reliability among the 3 observers for all measurements, with an ICC value of 0.91 (95% CI, 0.83-0.95) for the FRHD across all cadavers. Excellent reliability was also found for the FRHD in the cadavers with an intact LUCL and native radial head, with an ICC value of 0.79 (95% CI, 0.49-0.93); in the cadavers with a disrupted LUCL and native radial head, with an ICC of 0.82 (95% CI, 0.50-0.95); and in the cadavers with a disrupted LUCL and replacement radial head, with an ICC of 0.94 (95% CI, 0.83-0.98). The average difference in MRI measurements between observers for the FRHD in the coronal and sagittal planes was 0.2 mm. The average difference in fluoroscopic measurements between observers in the cadaveric FRHD for all specimens was 0.3 mm. With regard to intraobserver reliability, we again found excellent intraobserver reliability among the 3 observers for all measurements, with an ICC value of 0.92 (95% CI, 0.840.96) for the FRHD across all cadavers. In the cadavers with an intact LUCL and native radial head, we found an ICC value of 0.82 (95% CI, 0.50-0.95) for the FRHD; in the cadavers with a disrupted LUCL and native radial head, 0.90 (95% CI, 0.63-0.98); and in the cadavers with a disrupted LUCL and replacement radial head, 0.80 (95% CI, 0.45-0.94).
Discussion Sizing of radial head arthroplasty continues to be a clinical challenge and can lead to poor results when complicated by overlengthening.2,21,23 Several methods have been described to help with sizing the radial head prosthesis. Doornberg et al10 studied the relationship between the radial head and the proximal aspect of the lesser sigmoid notch after reviewing 17 computed tomography scans. While the radial head was on average 0.9 mm proximal to the proximal aspect of the lesser sigmoid notch, there was significant variability among the patients, with some patients’ radial heads sitting proximal to the lesser sigmoid notch and some radial heads sitting distal to the lesser sigmoid notch. Likewise, van Riet et al22 described the relationship between the radial head and the lesser sigmoid notch in cadaveric specimens. After resecting the radial
A.J. Lampley et al. head from the cadaveric specimens, they found that the resected radial head length was roughly equivalent to the distance from the radial neck cut to the proximal edge of the lesser sigmoid notch. Therefore, in the operating room, the measured length from the proximal radial neck to the proximal lesser sigmoid notch can be used to determine the size of the radial head. In another cadaveric experiment, Frank et al13 showed that the lateral ulnohumeral joint surfaces articulated without a gap when a correctly sized radial head prosthesis was implanted. When the radial head prosthesis was overstuffed by 2 mm or more, visible gapping was seen at the lateral ulnohumeral joint. However, it is important to note that Rowland et al19 reviewed 50 normal AP elbow radiographs and found that the lateral ulnohumeral joint space was wider than the medial ulnohumeral joint space on 94% of the radiographs. In addition, the lateral ulnohumeral joint space was 1 mm or greater in width than the medial ulnohumeral joint space in 32% of the normal elbows. Thus, the lateral ulnohumeral joint space is normally wider than the medial joint space, and this sign may not be a reliable marker of overlengthening the radial head prosthesis during radial head replacement. While these methods are valuable in implanting an appropriately sized radial head, they are difficult to apply as accurate intraoperative measurement is challenging, and it is difficult to visualize these medial landmarks once the trial or final prosthesis is implanted. Using MRIs, we demonstrated that the radiocapitellar synovial fold is a structure with a constant anatomic relationship to the radial head. Our measurements indicate that the plane defined by the proximal subchondral bone of the most proximal aspect of the radial head sits slightly less than 1 mm distal to the most prominent aspect of the radiocapitellar synovial fold in both the coronal and sagittal planes. Although there was a small degree of variability in this measurement between patients, the native radial head was always distal to the synovial fold. Similarly, this anatomic relationship was confirmed in our cadaveric specimens as the fold was present in all specimens and averaged 1.5 mm from the subchondral surface of the radial head on radiographs. However, it is important to note that the FRHD was measured from the subchondral bone on the radiographs, and the majority of this distance was likely made up of the thickness of the radial head articular cartilage. After the radial head was replaced, the FRHD was 0.5 mm. This decrease in distance to the fold represents the replacement of the radial head articular surface with the metal implant, which—unlike articular cartilage—can be measured on radiographs. Yeung et al25 showed cartilage thickness around the rim of the radial head to have regional variability, with cartilage thickness ranging from 0.8 to 1.7 mm. Applying the range of cartilage thickness to our data range, we can predict that the synovial fold sits on average 0.10.5 mm from the subchondral bone, which agrees well with our FRHD after radial head replacement. Furthermore, we reported an average FRHD of 0.9 mm after reviewing 30 MRIs of normal elbows, which also agrees with our data.
ARTICLE IN PRESS Novel radial head replacement reference point To determine whether the relationship between the radiocapitellar synovial fold and the radial head was affected by injury, we simulated significant soft-tissue damage to the lateral aspect of the humerus by stripping off all soft-tissue attachments on the lateral epicondyle and lateral ridge of the humerus to include the LUCL and the common extensor origin. This type of soft-tissue injury is frequently associated with comminuted radial head fractures,4 and this type of cadaveric model has been used in prior studies of this injury pattern.3,8,14,17 We found a statistically significant distal migration of the synovial fold by an average of 0.5 mm. While a simulated lateral soft-tissue injury did change the FRHD, a difference of less than 0.5 mm is likely not clinically significant as this small amount of change would have little impact on the sizing of the radial head arthroplasty. In fact, Van Glabbeek et al21 demonstrated that changes were seen in elbow kinematics only by overlengthening the radius by 2.5 mm or greater. The limitations of our study include a relatively small number of specimens. This cadaveric study attempts to re-create an LUCL injury with soft-tissue stripping, and it is unknown whether this accurately simulates the clinical scenario of terribletriad fracture-dislocation. However, this technique has been used in other cadaveric studies to simulate LUCL injury.3,8,14,17 Although our experience has shown the synovial fold to be visible and useful in the setting of trauma, further clinical studies will be helpful to define the usefulness of the synovial fold in radial head sizing in these clinical situations. We evaluated the use of the radiocapitellar synovial fold as a landmark to clinically help find the appropriate radial head replacement length. Both cadaveric and MRI evaluations confirmed that the radiocapitellar synovial fold is an anatomic structure with a constant anatomic relationship to the radial head. We also demonstrated that this anatomic relationship between the synovial fold and radial head is not significantly changed by a simulated LUCL injury in a cadaveric model. The proximity of the fold to the radial head confirms the potential benefit as a guide when sizing the radial head arthroplasty length.
Conclusion Our study is meant to supplement the existing sizing references. We believe the radiocapitellar synovial fold may be of significant utility as a radial head sizing landmark given that this is a posterolateral structure—both making it easily visualized while performing the lateral approach for the arthroplasty and allowing the proper plane for the radial head articular surface to be determined in conjunction with the medial landmarks. This study shows that the radiocapitellar synovial fold is on average 0.8-1.5 mm from the subchondral bone of the radial head and should always be proximal to the articular surface of the radial head. Using this information, the surgeon can prevent overstuffing as the implant should not be placed proximal to the synovial fold.
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Acknowledgment The authors thank Dr. Matthew Ramsey for providing the inspiration for this investigation.
Disclaimer This cadaveric study was completed with an educational grant in the amount of $19,000 from Acumed (Hillsboro, OR, USA). The outside source of funds was not involved in data collection, data analysis, or preparation or editing of the manuscript. The outside source of funds was used to fund the cadaveric specimens and implants used in the study. Marc J. Richard is a paid consultant for Acumed. All the 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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