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Canal preparation for prosthetic radial head replacement: rasping versus reaming
J Shoulder Elbow Surg (2013) 22, 1474-1479
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Canal preparation for prosthetic radial head replacement: rasping versus reaming Dave R. Shukla, MB, BCh, Decheng Shao, MD, PhD, James S. Fitzsimmons, BSc, Andrew R. Thoreson, MS, Kai-Nan An, PhD, Shawn W. O’Driscoll, MD, PhD* Department of Orthopedic Surgery, Biomechanics Laboratory, Mayo Clinic, Rochester, MN, USA Background: While many design-specific features of radial head prostheses have been studied (ie, geometry and surface coating), the optimum technique for canal preparation has not been determined. We hypothesized that preparation of the radial canal with a reamer would allow for the accommodation of a larger stem diameter versus following canal preparation with a rasp, and would provide acceptable stem-bone micromotion. Methods: Paired proximal radii from 7 cadavers were prepared by a rasp on one side and a reamer on the contralateral side. Cementless radial head stems of increasing diameter were sequentially implanted up to the maximum size or until a fracture occurred and the micromotion between the stem and bone was recorded. Results: In 3 of 5 pairs, at least a 1 mm larger stem size fit into the canal after reaming versus after rasping (P ¼ .04). 5 of 7 radii fractured secondary to intentional stem oversizing. For the optimally-sized stems, similar micromotion values were observed whether the canal was rasped (41 6 mm) or reamed (44 6 mm) (P ¼ .72). Discussion: This study investigated an aspect of radial head arthroplasty technique about which little has currently been published. It is possible that use of a reamer rather than a rasp, while providing similar initial stability, might expand the stem size options for initial press-fit stability, and decrease the risk of fracture. Conclusion: Radial canal preparation with a reamer allows for implantation of a 1 mm larger stem diameter versus rasping, while providing comparable initial stability to that achieved after rasping. Level of evidence: Basic Science Study, Biomechanics, Cadaver Model. Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Radial head; prosthesis; micromotion; arthroplasty; biomechanics; osteointegration
The Mayo Clinic IRB that convened on December 17, 2010 approved the project, entitled ‘‘Prosthetic Radial Head Stability’’: IRB protocol number 01-008186. *Reprint requests: Shawn W. O’Driscoll, MD, PhD, Department of Orthopedic Surgery, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA. E-mail address: [email protected] (S.W. O’Driscoll).
Optimization of the press-fit of cementless radial head stems is thought necessary for osteointegration and longevity.11,16 Satisfactory results have been reported for metallic radial head arthroplasty, though the follow-up in these studies is limited in relation to the survival of these implants.6,8,10,13 Aseptic loosening of the stem, which reportedly ranges from a mild periprosthetic lucency to symptomatic loosening leading to implant removal, has
1058-2746/$ - see front matter Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2013.06.015
Canal preparation for radial head prostheses been reported.6,8,10,13 O’Driscoll and Herald recently reported a series of 9 patients in whom revision surgery was required for painful, aseptic loosening of press-fit radial head implants.15 The effects of implant-specific factors such as design and geometry have been reported.1,7,12 However, little has been reported on the influence of technique used for radial canal preparation,17 and no data exists concerning the efficacy of different instrument types. Techniques currently used for preparation of the radial canal include rasping/broaching, reaming, and compaction. Though they primarily relate to cemented implants, scientific articles comparing the biomechanical effects of each have been reported as they relate to other types of arthroplasty, such for hip5,9 and knee4,14 replacements. DiGiovanni et al concluded that the case for reaming or broaching lacked sound data and comparative trials, while stating that specific comparative and basic science studies would be imperative to answer questions regarding technique and the degree of bone removal.5 To our knowledge, the biomechanical effects of various canal preparation techniques for radial head arthroplasty has not been investigated as of yet. We hypothesized that preparation of the radial canal with a reamer would allow for the implantation of a larger stem size before an iatrogenic fracture occurs than when using a rasp, and would provide acceptable initial implant stability as demonstrated by micromotion within the threshold for bone ingrowth.
Materials and methods Fourteen fresh frozen cadaveric elbows (7 pairs) were obtained from our institutional donor program and thawed overnight at room temperature. 4 pairs were from male donors and 3 were from female donors, their average age being 82 years (range, 64-100). The specimens were dissected by layer with removal of all soft tissues. The proximal third of the radius was resected and potted in an aluminum tube using polymethyl methacrylate. The cement level did not extend proximally enough to contact the bicipital tuberosity, the visualization of which was used to ensure consistent alignment. Each proximal radius was cemented with the long axis of the radial neck oriented vertically. A microsagittal saw was used to make a transverse cut (horizontal plane) for resection of the radial head at the head/neck junction. A fine jeweler’s saw, which was less than 1 mm thick, was used to create a 4-mm-long, vertical, full cortical-thickness notch in the lateral aspect of the radial neck. This was performed to mimic the clinical situation in which a residual crack extends below the level of neck resection.
Radial head implant The Anatomic Radial Head System (Acumed, Hillsboro, OR, USA) was tested in this study. The implant’s titanium stem was 25 mm long, grit-blasted, tapered, and fluted for rotational stability. A stem with a plus 4-mm neck was used in all specimens.
1475
Figure 1 Photograph showing the rasp and reamer that were used. Both instruments were manually inserted. This meant that the cantilever quotient was 0.35,18 which is within the acceptable range for implant stability. The available stem diameters ranged from 6-10 mm, in 1-mm increments, until the maximum size (10 mm) was used or a fracture occurred.
Canal preparation The rasps and reamers designed for canal preparation prior to stem insertion were 0.5 mm undersized compared to their designated stem sizes. For example, the size 8 rasp, used for an 8-mm stem, was 7.5 mm in diameter. The rasps are commercially available, while the reamers were custom-made for this study (Fig. 1).
Rasping procedure A stepwise technique was used to insert the rasp and then the stem, which was intended to simulate surgical implantation. A previouslyreported2,3,17,18 slap-hammer with removable weights was used for insertion of the rasp and stem. A marking on the rasp, which had been pre-designated by the manufacturer, indicated when full insertion had been achieved. The stem was considered to be fully inserted when the collar rested flush against the radial neck.
Reaming procedure A custom-made fixture was used to standardize the methodology to ensure consistency of the reaming process (Fig. 2). The platform onto which the fixture was secured was mounted on top of an X-Y stage. Alignment was made possible by the use of a customized articulating surface (large ball-bearing permitting multiple degrees of freedom - rotation, angulation and translations of the radius). The reamers were attached to a fixture (Fig. 2) that allowed a minimal amount of angulation while permitting free rotation of the instrument. A cross-bar that contained a bearing allowed the reamer to be supported, while allowing some free motion (ie, axial translation, rotation, and minimal angulation) during reaming. The potted specimen was tightly secured within a specimen grip, which was seated on top of an articulating joint. Motion at this joint allowed for alignment of the specimen such that the canal was oriented vertically. The joint was then tightened and immobilized. By allowing motion of the X-Y stage as well as the articulating joint, we were able to align the long axes of the reamer and the radial canal prior to tightening the mobile components of the device, thereby minimizing motion.
1476
D.R. Shukla et al. system. A second load cell-equipped pneumatic device applied an eccentric load of 10 N to a point 4.5 cm from the plate’s center. This load provided the bending moment (45 N$cm) which produced the micromotion. Vertical displacement of the plate was recorded by a mounted laser sensor at a point 4.5 cm from the stem’s center, opposite from the eccentric load. We were able to derive micromotion of the stem from displacement of the plate using simple geometry.
Micromotion measurements The stem micromotion testing sequence was done in a stepwise manner. Testing of every specimen was initiated with the smallest rasp or reamer (depending on experimental group) and stem diameter. Micromotion was measured following stem insertion. The stem was then removed and the next incrementally larger size of rasp or reamer was used to prepare the canal, followed by insertion of the corresponding stem. This sequence progressed until either the maximum stem size was reached (10 mm), or until a crack occurred in the radius. The rasp/reamer sizes were categorized in relation to the largest size stem (‘max’) that could be inserted without causing an iatrogenic fracture. This allowed for a uniform system of size comparison across specimens. If a crack did occur, that rasp/ reamer/stem size was labeled as ‘max þ 1’. The following is an example of the manner in which we categorized a specimen that experienced fracture at 9 mm: 6 mm (max – 2), 7 mm (max – 1), 8 mm (max), 9 mm (max þ 1).
Statistical analysis
Figure 2 Custom-made fixture used to standardize the reaming procedure. (a) bearing. (b) Proximal radius. (c) Specimen grip. (d) X–Y stage. An axial force ( f ) and torque (Ƭ ) were applied simultaneously. The specimen was reamed in a manner designed to simulate intra-operative technique. The reamer handle was turned manually 180 in a clockwise direction while applying an axial force. This cycle was repeated until the reamer was fully inserted, as indicated by a pre-designated groove. All specimens were reamed by the same person.
Micromotion testing A previously-reported device was used to measure micromotion.1-3,12,17,18 Following stem insertion, a metal plate (10mm thickness, 100-mm diameter) was fitted around the top of the stem and secured with a set screw. Rigid fixation of the plate-specimen construct was augmented by inserting it into a tightly-fitted steel sleeve, which itself was tightened using 2 collar clamps. Employing these extra measures ensured that the detected motion could not originate from any movement within the testing system, but would be due to motion between the stem and the bone. A 100-N, pneumatically-applied load delivered to the stem’s center simulated a joint compressive force, working to further eliminate the possibility of other motion within the
All data were reported as the mean standard error. The data were modeled with the use of 1-Factor and 2-Factor Repeated Measures Analysis of Variance, means contrast comparisons, and paired sign tests where appropriate, with a significance level of P .05. A power estimate indicated that with a sample size of 7 pairs we had 80% power to detect significant differences in micromotion between the rasped and reamed radii of 17 mm in this study.
Results Maximum stem diameter The maximum stem diameter which the proximal radius could accommodate without a fracture occurring differed significantly between the 2 groups. Reaming of the canals allowed the canal to accommodate a 1 mm larger stem size versus when the canals were rasped (P ¼ .04). Fractures occurred in 5 out of 7 pairs. In 3 of these 5 pairs, at least a 1 mm larger stem size fit into the canal after reaming versus after rasping. The term ‘at least’ is appropriate here, as in 2 of the 3 pairs in which reaming allowed for a larger stem size, the reamed radii did not fracture at all, while the rasped radii did fracture (Table I). There were no occurrences in which the reamed radii was not able to accommodate the same size or larger stem than the rasped radii.
Canal preparation for radial head prostheses
1477
Table I In 3 of the 5 pairs, in which fractures occurred, the reamed radii were able to accommodate stems of at least 1 mm or larger diameter (P ¼ .04) Pair #
Maximum stem diameter without fracture occurring Rasped radius
Reamed radius
1 3 4 6 7
8 9 8 9 8
8 mm >10 mm (no fracture) 9 mm >10 mm (no fracture) 8 mm
mm mm mm mm mm
D
0 1 1 1 0
Micromotion As previously documented, micromotion between the implant and bone decreased as larger-diameter stems were inserted (Fig. 3). The ‘oversized’ groups demonstrated significantly less micromotion than the ‘undersized’ groups, observed after both rasping (P ¼ .002) and reaming (P ¼ .007). The type of instrument (rasp versus reamer) used to prepare the canal did not influence micromotion of the prosthetic radial head stem. Micromotion of the optimally-sized stem (max) was 41 6 mm after the canal was rasped and 44 6 mm after the canal was reamed (P ¼ .72).
Discussion This study showed that in a model with a notched radial neck, preparation of the canal with a reamer allowed for a 1 mm larger stem size to be implanted without a longitudinal hoop-stress neck fracture occurring versus when the canal was rasped. The initial stability of the press-fit radial head stems implanted after canal reaming were similar to those implanted after rasping, and within the acceptable threshold (100 mm).2,3,17,18 To our knowledge, the effect of instrument type and surgical technique on fixation of cementless radial head implants has not been reported in the literature. These variables have been extensively studied for other types of arthroplasty, such as the hip5,9,14 and knee,4 as they are important aspects of joint replacement surgery. These data can contribute to the evolution of radial head implantation techniques, and should be taken into account by implant manufacturers. The observation that reaming allows for at least a 1 mm larger diameter stem size to be implanted versus than after rasping carries important clinical implications. It is likely that the proximal radius incurs less impact force during the reaming process versus during the rasping process, given the nature of each instrument’s required technique. For example, the work of reaming involves torque combined with a steady axial force, while rasping necessitates an impact force secondary to hammer strokes. Osteoporotic bone may be more prone to fracture
Figure 3 Micromotion values decreased as the stem size increased. Rasp preparation of the canal did not result in different micromotion values versus reamer preparation of the canal. Data shown are the means standard error of the means (error bars).
secondary to hoop-stresses during canal preparation. Additionally the proximal radius may be prone to propagation of microfractures following elbow trauma. As reaming the canal may exert less of an impact than rasping, reaming may be more advantageous in certain situations, and could theoretically reduce the risk of iatrogenic fracture in patients undergoing radial head replacement. The present study incorporated a small notch in the radial neck after radial head resection to mimic the clinical situation in which a fracture extends partially into the neck after the head is removed. Micromotion in the present study correlated favorably with a previous biomechanical study showing that initial stability of a textured radial head stem was not compromised by the presence of a crack in the radial neck.3 However, it should be noted that the radii in that study incurred a hoop-stress fracture during stem insertion secondary to hoop stresses, while the cortical discontinuity in the present study was created prior to implantation. The direction of implant loading in this study did not influence the measured micromotion (P ¼ .9). The implant was loaded in the anteromedial and lateral directions. Anteromedial loading was performed to simulate an eccentric load incurred by the implant during a posterolateral subluxation. For example, the capitellum contacts the anteromedial radial head during clinical posterolateral subluxation. Loading into the lateral aspect of the radial neck caused the stem to be loaded directly into the surgically created notch, such that assessment of the simulated fracture line’s effect on implant stability was possible. In each specimen, the loading direction occurred randomly at first, and then was systematically alternated throughout the sequential increase in stem size. For example, if the 6-mm stem was initially loaded laterally and then anteromedially, the 7-mm stem was first loaded anteromedially then laterally. A previous study employing the same testing model reported that loading direction did not influence stability of
1478 the stem.12 The data in the present study are similar to those in that prior study of micromotion in intact proximal radii (ie, no notch). The effects of reaming versus broaching have been reported for hip and knee arthroplasty,4,5,9,14 and primarily concerned cemented implants. Channer et al recorded initial stability of a tibial tray under a 100 N load following either reaming or canal dilation.4 They found that canal compaction provided better initial stability. Though our loading conditions were similar (100 N), our data differ in that stability (ie, micromotion) was not different between the 2 groups. This may be due to the fact that a radial head prosthesis relies more on stem fixation than support at the cut surface. Though it would be inappropriate to extrapolate conclusions for the radial head from studies performed on other joints, the existence of such other studies highlights the fact that such data are important technical aspects of radial head replacement and should be investigated further. This study was limited by its sample size of 7 pairs. However, a post-hoc power calculation demonstrated that with a sample size of 7 pairs, there was 80% power to detect a micromotion difference of 17 mm between the rasped and reamed radii (b ¼ 0.2, a ¼ 0.05). Additionally, only one implant design and one reamer design were tested. It is possible that a reamer with a different geometry could influence the results. A third limitation would be the manner in which the canals were reamed. A fixture was used to both guide the reamer and to align the specimen. However, in this initial study examining the influence of canal preparation instrument and technique, the use of the fixture allowed for standardization of the reaming process in an effort to minimize the variability between specimen preparation. A fourth limitation was that neither the in vivo effect of each technique nor the effect of the notch on bone ingrowth could be assessed. For example, it is conceivable that bone removal caused by reaming results in principally stem fixation in the mid-neck, which could perhaps increase stress shielding in the proximal neck. A reasonable future study could focus on the biomechanics of all available canal preparation techniques, including the use of a compactor, which was not evaluated here. The relatively older mean age (82 years) of the cadavers tested should be taken into account as well. Radial head replacement is typically performed in a younger population. The advanced age of the donors may render the specimens more prone to osteopenia and perhaps increased micromotion. Repeated testing of each specimen was carried out in sequence. For example, progressively larger instrument sizes (rasp or reamer) were inserted into the same radius. This may have affected the bone quality, and possibly the micromotion, of each subsequent testing cycle. However, the use of a new specimen for each testing cycle (ie, stem size) would not have been appropriate for several reasons. First, this would have greatly increased the cost of the study and consumption of limited resources. Second, repeated canal preparation using progressively increasing instrument
D.R. Shukla et al. diameters simulates what is performed in the clinical setting. Additionally, we would have been unable to accurately compare the relative decrease in micromotion between instrument/stem sizes if different specimens were being used.
Conclusion Radial canal preparation with a reamer allowed for the accommodation of at least a 1 mm larger cementless, textured stem versus preparation with a rasp. Additionally, the initial stability of press-fit radial head implants is within the threshold conducive to bone ingrowth after reaming the canal, and is comparable to that achieved after rasping.
Acknowledgment The prosthetic components used in this study were provided by Acumed, LLC.
Disclaimer One of the authors (SOD) and the Mayo Foundation receive royalties from commercial entities related to the subject of this article (Tornier, Acumed). The other authors, their immediate families, and any research foundation with which they are affiliated received no financial payments or other benefits from any commercial entity related to the subject of this article. There were no outside funding or grants received that assisted in this study.
References 1. Chanlalit C, Fitzsimmons JS, Moon JG, Berglund LJ, An KN, O’Driscoll SW. Radial head prosthesis micromotion characteristics: Partial versus fully grit-blasted stems. J Shoulder Elbow Surg 2011;20: 27-32. http://dx.doi.org/10.1016/j.jse.2010.05.030 2. Chanlalit C, Fitzsimmons JS, Shukla DR, An KN, O’Driscoll SW. Micromotion of plasma spray versus grit-blasted radial head prosthetic stem surfaces. J Shoulder Elbow Surg 2010;20:717-22. http://dx.doi. org/10.1016/j.jse.2010.11.010 3. Chanlalit C, Shukla DR, Fitzsimmons JS, An KN, O’Driscoll SW. Effect of hoop-stress fracture on micromotion of textured ingrowth stems for radial head replacement. J Shoulder Elbow Surg 2011;21: 949-54. http://dx.doi.org/10.1016/j.jse.2011.05.001 4. Channer MA, Glisson RR, Seaber AV, Vail TP. Use of bone compaction in total knee arthroplasty. J Arthroplasty 1996;11:743-9. 5. DiGiovanni CW, Garvin KL, Pellicci PM. Femoral preparation in cemented total hip arthroplasty: reaming or broaching? J Am Acad Orthop Surg 1999;7:349-57.
Canal preparation for radial head prostheses 6. 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 7. Ferreira LM, Stacpoole RA, Johnson JA, King GJ. Cementless fixation of radial head implants is affected by implant stem geometry: an in vitro study. Clin Biomech (Bristol, Avon) 2010;25:422-6. http://dx. doi.org/10.1016/j.clinbiomech.2010.02.001 8. 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 9. Ioannidis TT, Apostolou CD, Korres DS, Papaletsos I, Gandaifis ND, Panagopoulos CN, et al. Reaming versus broaching in cemented hip arthroplasty: mechanical stability in cadaver femora. Acta Orthop 2005;76:326-30. http://dx.doi.org/10.1080/ 00016470510030779 10. Knight DJ, Rymaszewski LA, Amis AA, Miller JH. Primary replacement of the fractured radial head with a metal prosthesis. J Bone Joint Surg 1993;75-B:572-6. 11. Ling RS. Observations on the fixation of implants to the bony skeleton. Clin Orthop Relat Res 1986:80-96.
1479 12. Moon JG, Berglund LJ, Domire Z, An KN, O’Driscoll SW. Stem diameter and micromotion of press fit radial head prosthesis: a biomechanical study. J Shoulder Elbow Surg 2009;18:785-90. http:// dx.doi.org/10.1016/j.jse.2009.02.014 13. Moro JK, Werier J, MacDermid JC, Patterson SD, King GJ. Arthroplasty with a metal radial head for unreconstructible fractures of the radial head. J Bone Joint Surg Am 2001;83-A:1201-11. 14. Newman MA, Bargar WL, Hayes DEJ, Taylor JK. Femoral canal preparation for cemented stems: reamers versus broaches. In: 60th American Academy of Orthopaedic Surgeons; 1993. San Francisco, CA. 15. O’Driscoll SW, Herald JA. Forearm pain associated with loose radial head prostheses. J Shoulder Elbow Surg 2012;21:92-7. http://dx.doi. org/10.1016/j.jse.2011.05.008 16. Pilliar RM, Lee JM, Maniatopoulos C. Observations on the effect of movement on bone ingrowth into porous-surfaced implants. Clin Orthop Relat Res 1986:108-13. 17. Shukla DR, Fitzsimmons JS, An KN, O’Driscoll SW. Effects of rasp mismatch on plasma spray radial head stems. J Shoulder Elbow Surg 2011;21:955-60. http://dx.doi.org/10.1016/j.jse.2011.05.009 18. Shukla DR, Fitzsimmons JS, An KN, O’Driscoll SW. Effect of stem length on prosthetic radial head micromotion. J Shoulder Elbow Surg 2012;21:1559-64. http://dx.doi.org/10.1016/j.jse.2011.11.025
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Canal preparation for prosthetic radial head replacement: rasping versus reaming Dave R. Shukla, MB, BCh, Decheng Shao, MD, PhD, James S. Fitzsimmons, BSc, Andrew R. Thoreson, MS, Kai-Nan An, PhD, Shawn W. O’Driscoll, MD, PhD* Department of Orthopedic Surgery, Biomechanics Laboratory, Mayo Clinic, Rochester, MN, USA Background: While many design-specific features of radial head prostheses have been studied (ie, geometry and surface coating), the optimum technique for canal preparation has not been determined. We hypothesized that preparation of the radial canal with a reamer would allow for the accommodation of a larger stem diameter versus following canal preparation with a rasp, and would provide acceptable stem-bone micromotion. Methods: Paired proximal radii from 7 cadavers were prepared by a rasp on one side and a reamer on the contralateral side. Cementless radial head stems of increasing diameter were sequentially implanted up to the maximum size or until a fracture occurred and the micromotion between the stem and bone was recorded. Results: In 3 of 5 pairs, at least a 1 mm larger stem size fit into the canal after reaming versus after rasping (P ¼ .04). 5 of 7 radii fractured secondary to intentional stem oversizing. For the optimally-sized stems, similar micromotion values were observed whether the canal was rasped (41 6 mm) or reamed (44 6 mm) (P ¼ .72). Discussion: This study investigated an aspect of radial head arthroplasty technique about which little has currently been published. It is possible that use of a reamer rather than a rasp, while providing similar initial stability, might expand the stem size options for initial press-fit stability, and decrease the risk of fracture. Conclusion: Radial canal preparation with a reamer allows for implantation of a 1 mm larger stem diameter versus rasping, while providing comparable initial stability to that achieved after rasping. Level of evidence: Basic Science Study, Biomechanics, Cadaver Model. Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Radial head; prosthesis; micromotion; arthroplasty; biomechanics; osteointegration
The Mayo Clinic IRB that convened on December 17, 2010 approved the project, entitled ‘‘Prosthetic Radial Head Stability’’: IRB protocol number 01-008186. *Reprint requests: Shawn W. O’Driscoll, MD, PhD, Department of Orthopedic Surgery, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA. E-mail address: [email protected] (S.W. O’Driscoll).
Optimization of the press-fit of cementless radial head stems is thought necessary for osteointegration and longevity.11,16 Satisfactory results have been reported for metallic radial head arthroplasty, though the follow-up in these studies is limited in relation to the survival of these implants.6,8,10,13 Aseptic loosening of the stem, which reportedly ranges from a mild periprosthetic lucency to symptomatic loosening leading to implant removal, has
1058-2746/$ - see front matter Ó 2013 Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2013.06.015
Canal preparation for radial head prostheses been reported.6,8,10,13 O’Driscoll and Herald recently reported a series of 9 patients in whom revision surgery was required for painful, aseptic loosening of press-fit radial head implants.15 The effects of implant-specific factors such as design and geometry have been reported.1,7,12 However, little has been reported on the influence of technique used for radial canal preparation,17 and no data exists concerning the efficacy of different instrument types. Techniques currently used for preparation of the radial canal include rasping/broaching, reaming, and compaction. Though they primarily relate to cemented implants, scientific articles comparing the biomechanical effects of each have been reported as they relate to other types of arthroplasty, such for hip5,9 and knee4,14 replacements. DiGiovanni et al concluded that the case for reaming or broaching lacked sound data and comparative trials, while stating that specific comparative and basic science studies would be imperative to answer questions regarding technique and the degree of bone removal.5 To our knowledge, the biomechanical effects of various canal preparation techniques for radial head arthroplasty has not been investigated as of yet. We hypothesized that preparation of the radial canal with a reamer would allow for the implantation of a larger stem size before an iatrogenic fracture occurs than when using a rasp, and would provide acceptable initial implant stability as demonstrated by micromotion within the threshold for bone ingrowth.
Materials and methods Fourteen fresh frozen cadaveric elbows (7 pairs) were obtained from our institutional donor program and thawed overnight at room temperature. 4 pairs were from male donors and 3 were from female donors, their average age being 82 years (range, 64-100). The specimens were dissected by layer with removal of all soft tissues. The proximal third of the radius was resected and potted in an aluminum tube using polymethyl methacrylate. The cement level did not extend proximally enough to contact the bicipital tuberosity, the visualization of which was used to ensure consistent alignment. Each proximal radius was cemented with the long axis of the radial neck oriented vertically. A microsagittal saw was used to make a transverse cut (horizontal plane) for resection of the radial head at the head/neck junction. A fine jeweler’s saw, which was less than 1 mm thick, was used to create a 4-mm-long, vertical, full cortical-thickness notch in the lateral aspect of the radial neck. This was performed to mimic the clinical situation in which a residual crack extends below the level of neck resection.
Radial head implant The Anatomic Radial Head System (Acumed, Hillsboro, OR, USA) was tested in this study. The implant’s titanium stem was 25 mm long, grit-blasted, tapered, and fluted for rotational stability. A stem with a plus 4-mm neck was used in all specimens.
1475
Figure 1 Photograph showing the rasp and reamer that were used. Both instruments were manually inserted. This meant that the cantilever quotient was 0.35,18 which is within the acceptable range for implant stability. The available stem diameters ranged from 6-10 mm, in 1-mm increments, until the maximum size (10 mm) was used or a fracture occurred.
Canal preparation The rasps and reamers designed for canal preparation prior to stem insertion were 0.5 mm undersized compared to their designated stem sizes. For example, the size 8 rasp, used for an 8-mm stem, was 7.5 mm in diameter. The rasps are commercially available, while the reamers were custom-made for this study (Fig. 1).
Rasping procedure A stepwise technique was used to insert the rasp and then the stem, which was intended to simulate surgical implantation. A previouslyreported2,3,17,18 slap-hammer with removable weights was used for insertion of the rasp and stem. A marking on the rasp, which had been pre-designated by the manufacturer, indicated when full insertion had been achieved. The stem was considered to be fully inserted when the collar rested flush against the radial neck.
Reaming procedure A custom-made fixture was used to standardize the methodology to ensure consistency of the reaming process (Fig. 2). The platform onto which the fixture was secured was mounted on top of an X-Y stage. Alignment was made possible by the use of a customized articulating surface (large ball-bearing permitting multiple degrees of freedom - rotation, angulation and translations of the radius). The reamers were attached to a fixture (Fig. 2) that allowed a minimal amount of angulation while permitting free rotation of the instrument. A cross-bar that contained a bearing allowed the reamer to be supported, while allowing some free motion (ie, axial translation, rotation, and minimal angulation) during reaming. The potted specimen was tightly secured within a specimen grip, which was seated on top of an articulating joint. Motion at this joint allowed for alignment of the specimen such that the canal was oriented vertically. The joint was then tightened and immobilized. By allowing motion of the X-Y stage as well as the articulating joint, we were able to align the long axes of the reamer and the radial canal prior to tightening the mobile components of the device, thereby minimizing motion.
1476
D.R. Shukla et al. system. A second load cell-equipped pneumatic device applied an eccentric load of 10 N to a point 4.5 cm from the plate’s center. This load provided the bending moment (45 N$cm) which produced the micromotion. Vertical displacement of the plate was recorded by a mounted laser sensor at a point 4.5 cm from the stem’s center, opposite from the eccentric load. We were able to derive micromotion of the stem from displacement of the plate using simple geometry.
Micromotion measurements The stem micromotion testing sequence was done in a stepwise manner. Testing of every specimen was initiated with the smallest rasp or reamer (depending on experimental group) and stem diameter. Micromotion was measured following stem insertion. The stem was then removed and the next incrementally larger size of rasp or reamer was used to prepare the canal, followed by insertion of the corresponding stem. This sequence progressed until either the maximum stem size was reached (10 mm), or until a crack occurred in the radius. The rasp/reamer sizes were categorized in relation to the largest size stem (‘max’) that could be inserted without causing an iatrogenic fracture. This allowed for a uniform system of size comparison across specimens. If a crack did occur, that rasp/ reamer/stem size was labeled as ‘max þ 1’. The following is an example of the manner in which we categorized a specimen that experienced fracture at 9 mm: 6 mm (max – 2), 7 mm (max – 1), 8 mm (max), 9 mm (max þ 1).
Statistical analysis
Figure 2 Custom-made fixture used to standardize the reaming procedure. (a) bearing. (b) Proximal radius. (c) Specimen grip. (d) X–Y stage. An axial force ( f ) and torque (Ƭ ) were applied simultaneously. The specimen was reamed in a manner designed to simulate intra-operative technique. The reamer handle was turned manually 180 in a clockwise direction while applying an axial force. This cycle was repeated until the reamer was fully inserted, as indicated by a pre-designated groove. All specimens were reamed by the same person.
Micromotion testing A previously-reported device was used to measure micromotion.1-3,12,17,18 Following stem insertion, a metal plate (10mm thickness, 100-mm diameter) was fitted around the top of the stem and secured with a set screw. Rigid fixation of the plate-specimen construct was augmented by inserting it into a tightly-fitted steel sleeve, which itself was tightened using 2 collar clamps. Employing these extra measures ensured that the detected motion could not originate from any movement within the testing system, but would be due to motion between the stem and the bone. A 100-N, pneumatically-applied load delivered to the stem’s center simulated a joint compressive force, working to further eliminate the possibility of other motion within the
All data were reported as the mean standard error. The data were modeled with the use of 1-Factor and 2-Factor Repeated Measures Analysis of Variance, means contrast comparisons, and paired sign tests where appropriate, with a significance level of P .05. A power estimate indicated that with a sample size of 7 pairs we had 80% power to detect significant differences in micromotion between the rasped and reamed radii of 17 mm in this study.
Results Maximum stem diameter The maximum stem diameter which the proximal radius could accommodate without a fracture occurring differed significantly between the 2 groups. Reaming of the canals allowed the canal to accommodate a 1 mm larger stem size versus when the canals were rasped (P ¼ .04). Fractures occurred in 5 out of 7 pairs. In 3 of these 5 pairs, at least a 1 mm larger stem size fit into the canal after reaming versus after rasping. The term ‘at least’ is appropriate here, as in 2 of the 3 pairs in which reaming allowed for a larger stem size, the reamed radii did not fracture at all, while the rasped radii did fracture (Table I). There were no occurrences in which the reamed radii was not able to accommodate the same size or larger stem than the rasped radii.
Canal preparation for radial head prostheses
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Table I In 3 of the 5 pairs, in which fractures occurred, the reamed radii were able to accommodate stems of at least 1 mm or larger diameter (P ¼ .04) Pair #
Maximum stem diameter without fracture occurring Rasped radius
Reamed radius
1 3 4 6 7
8 9 8 9 8
8 mm >10 mm (no fracture) 9 mm >10 mm (no fracture) 8 mm
mm mm mm mm mm
D
0 1 1 1 0
Micromotion As previously documented, micromotion between the implant and bone decreased as larger-diameter stems were inserted (Fig. 3). The ‘oversized’ groups demonstrated significantly less micromotion than the ‘undersized’ groups, observed after both rasping (P ¼ .002) and reaming (P ¼ .007). The type of instrument (rasp versus reamer) used to prepare the canal did not influence micromotion of the prosthetic radial head stem. Micromotion of the optimally-sized stem (max) was 41 6 mm after the canal was rasped and 44 6 mm after the canal was reamed (P ¼ .72).
Discussion This study showed that in a model with a notched radial neck, preparation of the canal with a reamer allowed for a 1 mm larger stem size to be implanted without a longitudinal hoop-stress neck fracture occurring versus when the canal was rasped. The initial stability of the press-fit radial head stems implanted after canal reaming were similar to those implanted after rasping, and within the acceptable threshold (100 mm).2,3,17,18 To our knowledge, the effect of instrument type and surgical technique on fixation of cementless radial head implants has not been reported in the literature. These variables have been extensively studied for other types of arthroplasty, such as the hip5,9,14 and knee,4 as they are important aspects of joint replacement surgery. These data can contribute to the evolution of radial head implantation techniques, and should be taken into account by implant manufacturers. The observation that reaming allows for at least a 1 mm larger diameter stem size to be implanted versus than after rasping carries important clinical implications. It is likely that the proximal radius incurs less impact force during the reaming process versus during the rasping process, given the nature of each instrument’s required technique. For example, the work of reaming involves torque combined with a steady axial force, while rasping necessitates an impact force secondary to hammer strokes. Osteoporotic bone may be more prone to fracture
Figure 3 Micromotion values decreased as the stem size increased. Rasp preparation of the canal did not result in different micromotion values versus reamer preparation of the canal. Data shown are the means standard error of the means (error bars).
secondary to hoop-stresses during canal preparation. Additionally the proximal radius may be prone to propagation of microfractures following elbow trauma. As reaming the canal may exert less of an impact than rasping, reaming may be more advantageous in certain situations, and could theoretically reduce the risk of iatrogenic fracture in patients undergoing radial head replacement. The present study incorporated a small notch in the radial neck after radial head resection to mimic the clinical situation in which a fracture extends partially into the neck after the head is removed. Micromotion in the present study correlated favorably with a previous biomechanical study showing that initial stability of a textured radial head stem was not compromised by the presence of a crack in the radial neck.3 However, it should be noted that the radii in that study incurred a hoop-stress fracture during stem insertion secondary to hoop stresses, while the cortical discontinuity in the present study was created prior to implantation. The direction of implant loading in this study did not influence the measured micromotion (P ¼ .9). The implant was loaded in the anteromedial and lateral directions. Anteromedial loading was performed to simulate an eccentric load incurred by the implant during a posterolateral subluxation. For example, the capitellum contacts the anteromedial radial head during clinical posterolateral subluxation. Loading into the lateral aspect of the radial neck caused the stem to be loaded directly into the surgically created notch, such that assessment of the simulated fracture line’s effect on implant stability was possible. In each specimen, the loading direction occurred randomly at first, and then was systematically alternated throughout the sequential increase in stem size. For example, if the 6-mm stem was initially loaded laterally and then anteromedially, the 7-mm stem was first loaded anteromedially then laterally. A previous study employing the same testing model reported that loading direction did not influence stability of
1478 the stem.12 The data in the present study are similar to those in that prior study of micromotion in intact proximal radii (ie, no notch). The effects of reaming versus broaching have been reported for hip and knee arthroplasty,4,5,9,14 and primarily concerned cemented implants. Channer et al recorded initial stability of a tibial tray under a 100 N load following either reaming or canal dilation.4 They found that canal compaction provided better initial stability. Though our loading conditions were similar (100 N), our data differ in that stability (ie, micromotion) was not different between the 2 groups. This may be due to the fact that a radial head prosthesis relies more on stem fixation than support at the cut surface. Though it would be inappropriate to extrapolate conclusions for the radial head from studies performed on other joints, the existence of such other studies highlights the fact that such data are important technical aspects of radial head replacement and should be investigated further. This study was limited by its sample size of 7 pairs. However, a post-hoc power calculation demonstrated that with a sample size of 7 pairs, there was 80% power to detect a micromotion difference of 17 mm between the rasped and reamed radii (b ¼ 0.2, a ¼ 0.05). Additionally, only one implant design and one reamer design were tested. It is possible that a reamer with a different geometry could influence the results. A third limitation would be the manner in which the canals were reamed. A fixture was used to both guide the reamer and to align the specimen. However, in this initial study examining the influence of canal preparation instrument and technique, the use of the fixture allowed for standardization of the reaming process in an effort to minimize the variability between specimen preparation. A fourth limitation was that neither the in vivo effect of each technique nor the effect of the notch on bone ingrowth could be assessed. For example, it is conceivable that bone removal caused by reaming results in principally stem fixation in the mid-neck, which could perhaps increase stress shielding in the proximal neck. A reasonable future study could focus on the biomechanics of all available canal preparation techniques, including the use of a compactor, which was not evaluated here. The relatively older mean age (82 years) of the cadavers tested should be taken into account as well. Radial head replacement is typically performed in a younger population. The advanced age of the donors may render the specimens more prone to osteopenia and perhaps increased micromotion. Repeated testing of each specimen was carried out in sequence. For example, progressively larger instrument sizes (rasp or reamer) were inserted into the same radius. This may have affected the bone quality, and possibly the micromotion, of each subsequent testing cycle. However, the use of a new specimen for each testing cycle (ie, stem size) would not have been appropriate for several reasons. First, this would have greatly increased the cost of the study and consumption of limited resources. Second, repeated canal preparation using progressively increasing instrument
D.R. Shukla et al. diameters simulates what is performed in the clinical setting. Additionally, we would have been unable to accurately compare the relative decrease in micromotion between instrument/stem sizes if different specimens were being used.
Conclusion Radial canal preparation with a reamer allowed for the accommodation of at least a 1 mm larger cementless, textured stem versus preparation with a rasp. Additionally, the initial stability of press-fit radial head implants is within the threshold conducive to bone ingrowth after reaming the canal, and is comparable to that achieved after rasping.
Acknowledgment The prosthetic components used in this study were provided by Acumed, LLC.
Disclaimer One of the authors (SOD) and the Mayo Foundation receive royalties from commercial entities related to the subject of this article (Tornier, Acumed). The other authors, their immediate families, and any research foundation with which they are affiliated received no financial payments or other benefits from any commercial entity related to the subject of this article. There were no outside funding or grants received that assisted in this study.
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1479 12. Moon JG, Berglund LJ, Domire Z, An KN, O’Driscoll SW. Stem diameter and micromotion of press fit radial head prosthesis: a biomechanical study. J Shoulder Elbow Surg 2009;18:785-90. http:// dx.doi.org/10.1016/j.jse.2009.02.014 13. Moro JK, Werier J, MacDermid JC, Patterson SD, King GJ. Arthroplasty with a metal radial head for unreconstructible fractures of the radial head. J Bone Joint Surg Am 2001;83-A:1201-11. 14. Newman MA, Bargar WL, Hayes DEJ, Taylor JK. Femoral canal preparation for cemented stems: reamers versus broaches. In: 60th American Academy of Orthopaedic Surgeons; 1993. San Francisco, CA. 15. O’Driscoll SW, Herald JA. Forearm pain associated with loose radial head prostheses. J Shoulder Elbow Surg 2012;21:92-7. http://dx.doi. org/10.1016/j.jse.2011.05.008 16. Pilliar RM, Lee JM, Maniatopoulos C. Observations on the effect of movement on bone ingrowth into porous-surfaced implants. Clin Orthop Relat Res 1986:108-13. 17. Shukla DR, Fitzsimmons JS, An KN, O’Driscoll SW. Effects of rasp mismatch on plasma spray radial head stems. J Shoulder Elbow Surg 2011;21:955-60. http://dx.doi.org/10.1016/j.jse.2011.05.009 18. Shukla DR, Fitzsimmons JS, An KN, O’Driscoll SW. Effect of stem length on prosthetic radial head micromotion. J Shoulder Elbow Surg 2012;21:1559-64. http://dx.doi.org/10.1016/j.jse.2011.11.025