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A biomechanical assessment of fixation methods for a coronoid prosthesis
Clinical Biomechanics 32 (2016) 14–19
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Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
A biomechanical assessment of fixation methods for a coronoid prosthesis Alia B. Gray ⁎, Bashar Alolabi, Simon Deluce, Louis M. Ferreira, George S. Athwal, Graham J.W. King, James A. Johnson Hand and Upper Limb Centre, University of Western Ontario, St. Joseph's Health Centre, 268 Grosvenor St., London, ON N6A 4L6, Canada
a r t i c l e
i n f o
Article history: Received 19 June 2015 Accepted 25 November 2015 Keywords: Coronoid Elbow Biomechanics Prosthesis Fracture
a b s t r a c t Background: The coronoid process is an integral component for maintaining elbow joint stability. When fixation of a fracture is not possible, prosthetic replacement may be a feasible solution for restoring stability. The purpose of this in-vitro biomechanical study was to compare fixation methods for a coronoid implant. Methods: A coronoid prosthesis was subjected to distally-directed tip loading after implantation using four fixation methods: press-fit, anterior-to-posterior screws, posterior-to-anterior screws, and cement. Testing was performed on seven fresh-frozen ulnae in a repeated-measures model. Rounds of cyclic loading were applied at 1 Hz, for 100 cycles, increased in 50 N increments up to a maximum of 400 N. Micro-motion of the implant was quantified using an optical-tracking system. Outcome variables included total displacement, distal translation, gapping, anterior translation and axial stem rotation. Findings: Cement fixation reduced implant micro-motion compared to screw fixation, while the greatest implant micro-motion was observed in press-fit fixation. Comparing screw-fixation techniques, posterior–anterior screws provided superior stability only in distal translation. The implant did not experience displacements exceeding 0.9 mm with screw or cement fixation. Interpretation: Cement fixation provides the best initial fixation for a coronoid implant. However, the stability provided by both methods of screw fixation may be sufficient to allow osseous integration to be achieved for long-term fixation. Large displacements were observed using the press-fit fixation technique, suggesting that modifications would need to be developed and tested before this technique could be recommended for clinical application. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Traumatic elbow injuries can lead to significant elbow instability and disability. The coronoid process, which is a bony triangular eminence that projects from the anterior portion of the proximal ulna, is an integral component for maintaining elbow joint stability. This structure may be damaged in the setting of a traumatic injury, and given its importance for stability, Type II and larger coronoid fractures should be treated surgically with open reduction and internal fixation (ORIF) (Pollock et al., 2009a,b). While relatively uncommon, larger comminuted fractures that cannot be managed by ORIF pose a challenge in restoring elbow stability and can result in significant patient disability. The results of coronoid reconstruction with allografts and autografts have
⁎ Corresponding author at: 386 Yonge St. #1610, Toronto, ON M5B0A5, Canada. E-mail addresses: [email protected] (A.B. Gray), [email protected] (B. Alolabi), [email protected] (S. Deluce), [email protected] (L.M. Ferreira), [email protected] (G.S. Athwal), [email protected] (G.J.W. King), [email protected] (J.A. Johnson).
http://dx.doi.org/10.1016/j.clinbiomech.2015.11.017 0268-0033/© 2015 Elsevier Ltd. All rights reserved.
been historically poor, emphasizing the need for a reliable prosthetic device to adequately manage coronoid deficiencies (Chung et al., 2007; Van Riet et al., 2005). When ORIF is precluded due to comminution or osteopenia, a coronoid prosthesis may be a viable solution to restore elbow stability. Presently, there are no prosthetic devices of this nature available to address this clinical scenario. Biomechanical investigations have shown that initial stability of the implant is essential for proper fixation and osseous integration, which reduces the risk of component loosening and prolongs implant survival (Pilliar et al., 1986; Soballe et al., 1992). Excessive implant micro-motion has been shown to promote the ingrowth of fibrous tissue, which compared to bone ingrowth is insufficient to ensure long-term stability (Collier et al., 1988; Kienapfel et al., 1999; Pilliar et al., 1986; Soballe et al., 1992). The resulting aseptic loosening is the most common reason for performing revision surgeries of elbow prostheses, with revisions being typically more technically challenging, time consuming, costly, and having a higher incidence of complications than the initial joint replacement (Barrack, 1995; Barrack et al., 1995; Jafari et al., 2010; Kamineni & Morrey, 2004; Lavernia et al., 1995; Loebenberg et al., 2005; Ulrich et al., 2008).
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To our knowledge, no previous studies have examined the optimal fixation method for a prosthetic replacement for the coronoid process. Previous bio-mechanical investigations have shown that coronoid prostheses can restore stability to coronoid deficient cadaveric elbows. As such, examining potential fixation methods for implementation of such a device would be useful prior to clinical use. Press-fit cementless fixation, screw fixation, and cement fixation are all feasible options which should be investigated (Alolabi et al., 2011; Gray et al., 2013). Therefore, the purpose of this study was to determine the optimal fixation method of securing a coronoid prosthesis in the proximal ulna. We hypothesized that cement fixation would minimize the micro-motion of a coronoid prosthesis, while the greatest displacement would occur with the press-fit method. 2. Methods
ulna. The screw threads did not engage the prosthesis, thus achieving lag screw fixation. The AP screw holes were widened to accommodate larger machine screws (UNF#5-40, D = 3.18 mm) for PA fixation. These were passed anteriorly through the posterior surface of the ulna, through the prosthesis, and then were secured on its anterior surface with two stainless steel nuts. The screw threads did not engage the bone or the prosthesis. AP and PA screw fixation schematics are shown in Fig. 2. After the press fit and screw fixation techniques were tested, a rotary milling tool was used to widen the void in the metaphysis of the proximal ulna to create space for surgical cement (Simplex™ P bone cement, Stryker, Hamilton, ON, Canada). The opening in the fracture surface was then filled with cement, and the stem of the prosthesis was pressed into place and held stationary until the cement hardened. 2.3. Prosthesis tracking and loading protocol
2.1. Prosthesis design Morphological dimensions of the coronoid process from 18 cadaveric specimens (mean age 64.4 years [range 42 to 90 years], left arms: 11, right arms: 7, male arms: 11, female arms: 7) (Table 1) were measured from computed tomography scans (Mimics®: Materialise BV, Leuven, Belgium). A coronoid prosthesis was developed based on the following dimensions of the coronoid process: height, proximal-distal depth, medial-lateral width, and facet angles. Coronoid cartilage thickness was incorporated into the prosthesis design using the results from a parallel study conducted in our laboratory (Rafehi et al., 2011). The implant was fabricated from stainless steel using CAD–CAM technologies. Two threaded holes in the distal surface of the implant (Fig. 1) were used to attach and secure an optical tracking device, in order to monitor its motion relative to the ulna. Two through holes in the stem accommodate screws for antero-posterior (AP) and postero-anterior (PA) fixation methods. 2.2. Specimen preparation and mechanical test setup The soft tissues were removed from seven fresh frozen ulnae (mean age 79 years [range 60 to 88 years], left arms: 5, right arms: 2, male arms: 6, female arms: 1) (Table 1) and the distal sections of the bones were potted in bone cement. Digital calipers were used to measure the height of each coronoid process, and an oscillating sagittal saw was used to simulate a 40% (Type II) transverse coronoid fracture that is commonly encountered clinically (Doornberg et al., 2006; Regan & Morrey, 1989). The coronoid prosthesis was secured to the bone using the following four techniques in order: press-fit method, AP screws, PA screws, and cement. After the 40% coronoid osteotomy, a cavity slightly smaller than the implant stem was created in the cancellous bone located in the central region of the fractured surface. Press-fit fixation was accomplished by impacting the stem of the implant into the cavity. Two 2.4 mm cortical screws (Synthes, Missisauga, ON, Canada) were used for AP fixation. A 1.8 mm drill bit was used to drill holes directed posteriorly through the fracture surface, exiting through the posterior surface of the proximal ulna. The screws were then passed through the implant and securely fixed into the cortical bone of the posterior Table 1 Specimen data. The number, age, sex, and relative number of right and left specimens used for both the morphological (n = 18) and biomechanical study (n = 7).
Number of specimens (N) Left/right Male/female Mean age/range (years)
15
Morphological study
Biomechanical study
18 11/7 11/7 64 (42–90)
7 5/2 6/1 79 (60–88)
Optical trackers (Optotrak Certus®, NDI, Waterloo, ON, Canada) were mounted on the shaft of the ulna and on the prosthetic device to quantify motion of the prosthesis with respect to the ulna (Fig. 3). A stylus attached to a third optical tracker was used to digitize three points on the base of the articular face of the prosthesis (medial, ridge, and lateral) to monitor the micro-motion of the prosthesis throughout the testing protocol. The greater sigmoid notch and two flat points on the posterior surface of the ulna were also digitized in order to generate an anatomical coordinate system (shown in Fig. 4 with the three types of motion described in this study) for each specimen using a method previously described (McDonald et al., 2011; Sabo et al., 2011). Relative motions were determined with respect to the anatomical planes of the body. The four fixation methods were subjected to mechanical testing via a materials testing machine (Instron 8501®, Instron, Canton, MA, USA). Load was applied to the coronoid prosthesis using a custom designed fixture, developed to fit congruently against the medial and lateral facets of the implant. The load applicator was used to simulate loading imparted by the distal humerus. Distal loading was applied to the ridge of the implant's articular surface, midway between the base and tip of the prosthesis. Fixation methods were tested sequentially, first using the press-fit method, followed by AP, PA, and then cement fixation. Cyclic loading was applied to the prosthesis for eight cyclic loading regiments. Maximum cyclic loads ranged from 50 to 400 N in increments of 50 N, where each regiment lasted for 100 cycles at 1 Hz. Testing continued to the end of the loading protocol, or to the point of failure, which was defined as a displacement in any direction exceeding 2 mm at any one of the three digitized prosthesis points. The loading protocol used in this biomechanical study was based on the typical loads experienced within the human elbow. The maximum elbow flexion strength has been shown to occur at 90°, where a force of approximately three times the weight of the body has been estimated to pass through the elbow with heavy lifting (Kai-Nan & Morrey, 2000). Given this information, a force of approximately 2000 N may in some highly aggressive scenarios, be experienced in the elbow joint. The ulnohumeral joint is expected to experience 800 N of this force, based on the work of Halls and Travill, who reported that an applied axial force is distributed across the joint with 40% crossing the ulnohumeral joint and 60% crossing the radiohumeral articulation (Halls & Travill, 1964). This estimate of 800 N across the ulnohumeral joint is a maximum loading case; unlikely to be experienced by a patient postoperatively and would be distributed over the entire articular surface of the greater sigmoid notch. Applying a force of this magnitude to only the anterior 40% of the coronoid process would be well in excess of the regular physiological load experiences in this area, as such, the maximum load for this study was reduced to 400 N, approximately half of the maximum potential load, to more accurately represent potential loads experienced by patients in post-surgical rehabilitation,
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Fig. 1. The coronoid prosthesis. An isometric view of the coronoid prosthesis (a) used in the biomechanical testing is shown with anatomical orientation highlighted, where A indicates anterior, P indicates proximal, and M/L indicates medial or lateral. Two threaded holes in the distal surface of the prosthesis were used to attach and secure an optical tracking device to monitor its motion relative to the ulna, and two through holes through the stem accommodated antero-posterior (AP) and postero-anterior (PA) fixation methods. The stem used for press fit and cement fixation is highlighted in image d. Images b) c) and d) show further views of the implant from the distal aspect, bottom, and side profile respectively.
where extreme positions and excessive elbow stresses are avoided (Nirschl & Robert, 2000). Budoff et al. employed a similar loading protocol, where the coronoid was loaded posteriorly up to 445 N, in a study investigating screw and plate fixation methods for coronoid reconstruction (Budoff et al., 2011). 2.4. Outcome measures (dependent variables) and statistical analyses The total 3-dimensional displacement of the prosthesis was quantified for each fixation method to describe prosthesis micro-motion.
Three additional types of motion were analyzed to describe the behavior of the prosthesis with reference to the anatomical planes of the body; distal translation (Δd), gapping (Δg) (the anterior translation of the ridge point), and axial rotation (Δα) (rotation about the anterior– posterior axis). Distal translation was determined by tracking the displacement of the digitized ridge point parallel to the proximal–distal axis of the anatomical coordinate system, describing the motion of the prosthesis in the direction of the applied load. Articular gapping was measured by tracking the motion of the ridge point parallel to the anterior–posterior axis. The total displacement of the ridge point in 3-
Fig. 2. Coronoid prosthesis screw fixation. Anterior–posterior and posterior–anterior screw fixation methods are shown above. Two 2.4-mm cortical screws were used for anterior–posterior fixation as illustrated in (a) medial and (b) anterior views. Screws were passed through the body of the prosthesis and securely fastened into the cortical bone on the posterior surface of the ulna. For posterior–anterior fixation, two machine screws were passed through the bone and prosthesis, and secured on its anterior surface with two stainless steel nuts, illustrated in (c) medial and (d) anterior views.
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distal translation) throughout the duration of the loading protocol, categorized at the peak load levels (50 N, 100 N, 150 N, 200 N, 250 N, 300 N, 350 N, and 400 N). The press-fit fixation method was compared with the other methods throughout an abbreviated range of peak loads (50 N–150 N) as it did not consistently survive the entire testing protocol. 3. Results 3.1. Total displacement In a comparison of press-fit fixation with the other three fixation methods, the total displacement was found to be significantly less when the prosthesis was fixed with PA screws (P = 0.006), AP screws (P = 0.009), and cement (P = 0.004). Total displacement with cement fixation was found to be significantly less than the displacement for both PA and AP screws (P = 0.02). Total displacement was less with the PA verses the AP screws but this was not statistically different (P = 0.07) (Fig. 5a). 3.2. Distal translation
Fig. 3. Prosthesis motion with respect to coordinate axes. Two optical trackers were used to track prosthesis micro-motion relative to the stationary ulna, one secured to the coronoid prosthesis and another to the ulna. The loading direction is indicated by the vertical arrow, with the implant fixed with anterior–posterior screws.
dimensions, without reference to the anatomical coordinate system, was used as a failure criterion (N2 mm). Statistical comparisons between the fixation methods consisted of 2-way (peak load and fixation method) repeated-measures analysis of variance with significance defined at P b 0.05. Outcomes measures were the three different types of motion (axial rotation, gapping, and
The distal translation of the press-fit prosthesis was greater than both PA fixation (P = 0.001) and cement (P = 0.001). The distal translation of the press-fit method was greater than the AP fixation method, however this did not reach statistical significance (P = 0.09). Less distal translation was observed when the prosthesis was secured with cement fixation than either PA screw fixation (P = 0.04) or AP screw fixation (P = 0.01). The direction of screw fixation was also found to have a significant effect on the motion of the prosthesis in response to an applied load, where more translation was observed with AP screws in comparison to the PA screws (P = 0.04) (Fig. 5b). 3.3. Gapping displacement Significantly less prosthesis gapping was observed when the prosthesis was fixed with PA screws (P = 0.006), AP screws (P = 0.009), and cement (P = 0.006) than when press-fit fixation was employed. There was no difference in prosthesis gapping between the two screw fixation methods or cement fixation (P N 0.06) (Fig. 5c). 3.4. Axial rotation No differences were detected when the axial rotation of the prosthesis in press-fit fixation was compared to PA or AP fixation (P = 0.07 and P = 0.4 respectively). When cement was used to fix the prosthesis, less rotational motion was observed in comparison to the press-fit method (P = 0.02). Less rotation was observed when the prosthesis was secured with cement fixation as compared to both PA screw fixation (P = 0.03) and AP screw fixation (P = 0.02). No significant difference in rotational motion was detected between the two methods of screw fixation (P = 0.6) (Fig. 5d). 4. Discussion
Fig. 4. Prosthesis motion with respect to coordinate axes. In addition to total prosthesis displacement, three supplemental types of motion, which described prosthesis displacement and rotation with respect to the anatomical planes of the body, were measured. Distal translation (Δd) described motion of the prosthesis parallel to the proximal–distal axis. Gapping (Δg) described motion of the prosthesis parallel to the anterior–posterior axis; a displacement in this direction indicated a separation of the base of the prosthesis from the simulated fracture plane, causing a gap in the articular surfaces. Axial rotation (Δα) described the rotational motion of the prosthesis about the anterior–posterior axis.
As loosening is the most common cause for revision following elbow joint replacement surgery, establishing the optimal method for coronoid prosthesis fixation is useful clinically (Kamineni & Morrey, 2004; Loebenberg et al., 2005; Ulrich et al., 2008). As there are currently no commercially available prosthetic devices for the coronoid process, there is little available in the literature for comparison purposes. Previous biomechanical studies have reported reduced micro-motion and increased initial fixation strength with cement fixation (Bicknell et al., 2003; Jazrawi et al., 2001; Zheng et al., 2009). Our results generally agree with these findings, as cement fixation reduced prosthesis micro-motion when distal translation, axial rotation, and total motion
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Fig. 5. Prosthesis micro-motion in response to cyclic loading. The mean (+1 SD) displacement or rotation for the four fixation methods in response to cyclic loading for (a) total prosthesis displacement, (b), distal translation, (c) gapping displacement, and (d) axial rotation. The press-fit fixation method frequently reached only 150 N of the loading protocol, while cement and screw fixations consistently reached 400 N. The greatest prosthesis micro-motion was observed with press-fit fixation (P b 0.02), however no significant differences were found in axial rotation when press-fit fixation was compared to both types of screw fixation (P N 0.4), or when press-fit fixation was compared to anterior–posterior screws in distal translation (P N 0.09). Cement fixation largely minimized prosthesis micro-motion in comparison to both types of screw fixations (P b 0.03), with the exception of gapping displacement, where screw and cement fixation methods were not found to differ significantly (P N 0.3). Posterior–anterior screw fixation was found to provide superior stability in distal translation (P b 0.04) compared to anterior–posterior, however no significant differences were observed in the other three types of prosthesis motion (P N 0.05). The prosthesis did not experience displacements exceeding 0.9 mm in screw and cement fixations during the loading protocol.
were examined. Gapping motion was found to be quite minimal, and was similar in cement and screw fixation, not exceeding 0.2 mm. These small displacements reflect positively on both screw and cement fixation, suggesting only a small degree of separation is experienced between the articular surfaces of the prosthesis and coronoid, even at larger load magnitudes (400 N). The rotational and translational motion of the prosthesis was found to be quite minimal in general (less than 1° and 0.6 mm, respectively), reflecting positively on the future clinical application of a coronoid prosthesis, as it appears as though the original position of the prosthesis can be sufficiently maintained. Based on the results of this study, cement fixation of a coronoid prosthesis should provide secure initial fixation, particularly in older patients where uncemented fixation may be less successful due to poor bone quality. The results of this study suggest substantial micro-motion of the prosthesis can be expected if the prosthesis is fixed using a press-fit method. Furthermore, press-fit fixation could not consistently survive the loading protocol, further highlighting the inferior nature of this method. Collectively, this data suggests that press-fit fixation, using the prototype prosthesis in elderly cadavers, is not optimal for reducing micromotion, and would likely not provide sufficient stabilization required for osseous ingrowth (Kienapfel et al., 1999; Pilliar et al., 1986). In comparing the two screw fixation methods, the only significant difference noted was in distal translation, where PA screw orientation resulted in significantly less micro-motion. Overall, the magnitudes of the differences in micro-motion between screws were relatively small (b0.3 mm for distal translation), and may not be relevant clinically. The reliable nature of the PA screws, which does not depend on bone quality, and the ease of clinical application suggest this may be the preferred fixation option. Weaknesses of this study include the design of the prosthetic device, which consisted of a smooth press-fit stem, and did not include special
reamers and an oversized roughened stem. The stem design in this study was based on the anthropometric measurements of the 18 ulnae included in this study, further measurements from a larger sample population would be required to design a series of stem sizes appropriate for clinical use. We would also point out that the results of this study do not directly predict the response of long-term loading and in-vivo performance in response to repeated loading. While the relative strengths of the four fixation methods were quantified in this biomechanical study, and clinically significant loads were received by the implant with minimal micro-motion in response, the loading model used may not represent clinical failure. Furthermore, additional cycles, in excess of the 100 per loading step, may have been beneficial, as lower failure loads may have been observed. Although a small sample size was used in this study, the clear statistically significant findings indicate it was sufficient. A larger sample size may be useful however, to further differentiate between the relative strength of the two screw fixation methods. With respect to the modeling of load direction, when the elbow is flexed at 90°, the resulting load vector within the greater sigmoid notch has been shown to point somewhat posteriorly (Bicknell et al., 2003). In our model, the prosthesis was loaded in a direction parallel to the flat spot of the ulna. Although our model does not replicate the loading experienced physiologically in the elbow joint, it represents a worst-case scenario of prosthesis loading. Other limitations include the use of isolated denuded ulnae in the place of an intact joint. It is however likely that our results overestimated, rather than under-estimated, the magnitude of micromotion of the prosthesis, as the musculature and ligamentous structures of the elbow have been shown to contribute stabilizing effects to the joint (Fornalski et al., 2003). Second, it was not possible to randomize the order of the methods of prosthesis fixation. The effects of previous fixation could have been avoided by using separate cadaveric specimens
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for each method, however because of our preference to control for bone quality, fixation was performed repeatedly within the same specimen. As a result, the strength of the fixation methods performed later in the testing protocol may have been under-estimated, as the bone may have been weakened throughout testing. The repeated measures study design can also be considered a strength of this study, as it ensured the bone density was consistent for each fixation method. This study focuses on determining both the relative strengths of the fixation methods examined, as well as whether clinically significant loads can be withstood. Future studies should be conducted with a single fixation method per bone to more accurately determine accepted and failure loads. Strengths of this study include the quantification of the total 3dimensional motion of the prosthesis as well as specific types of motion using a state of the art optical tracking system. With this method we were able to isolate the displacement of the prosthesis with respect to the stationary ulna, unlike typical studies of this nature measuring crosshead displacement, which confound deformation of the bone and loading system. The cyclic loading protocol used, rather than constant loading, may be considered an advantage of this study, as it is a more clinically relevant method of load application, and thus more representative of the loading conditions experienced in the human body. Furthermore, a worst-case scenario was represented in this biomechanical study. The mean age of the cadaveric specimens at age 76 (range 60–84) represents a poorer bone quality than would be expected in the typical fracture patient, as such the strength of the fixation methods in these specimens was likely under-estimated. Had cadaveric specimens with higher bone quality been used, it is possible that even greater forces may have been accepted by each fixation method. The loading direction and magnitude chosen in this biomechanical study, where 400 N was applied to the tip of the coronoid, also likely overestimates the amount of force which would be physiologically experienced in a post operative patient. Although the use of older cadaver specimens does not represent the typical fracture patient, the use of poorer bone quality and high forces as described across the elbow joint results in a worstcase scenario. This may be considered a strength of the study, as it ensures the fixation methods were not over-estimated and suggests that both cement and screw fixation methods can truly withstand clinically relevant forces. The novel nature of this study, where the strengths of different fixation methods are compared may be considered both a strength, and a weakness, as at present, there are no similar gold standard studies in existence for comparison. 5. Conclusion In conclusion, micro-motion of a coronoid prosthesis was minimized when the prosthesis was fixed using cement in comparison to the other fixation techniques. Only small deviations from the original position of the prosthesis were observed with screw fixation, suggesting screw fixation could be a reasonable method for initial implant fixation. Large translational displacements were observed using the press-fit fixation technique employed in this study, suggesting modifications would need to be developed and tested before this could be recommended for clinical application. Acknowledgments We would like to acknowledge CIHR (The Canadian Institute of Health Research Operating Grant MOP- 106500) for funding contributions.
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A biomechanical assessment of fixation methods for a coronoid prosthesis Alia B. Gray ⁎, Bashar Alolabi, Simon Deluce, Louis M. Ferreira, George S. Athwal, Graham J.W. King, James A. Johnson Hand and Upper Limb Centre, University of Western Ontario, St. Joseph's Health Centre, 268 Grosvenor St., London, ON N6A 4L6, Canada
a r t i c l e
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Article history: Received 19 June 2015 Accepted 25 November 2015 Keywords: Coronoid Elbow Biomechanics Prosthesis Fracture
a b s t r a c t Background: The coronoid process is an integral component for maintaining elbow joint stability. When fixation of a fracture is not possible, prosthetic replacement may be a feasible solution for restoring stability. The purpose of this in-vitro biomechanical study was to compare fixation methods for a coronoid implant. Methods: A coronoid prosthesis was subjected to distally-directed tip loading after implantation using four fixation methods: press-fit, anterior-to-posterior screws, posterior-to-anterior screws, and cement. Testing was performed on seven fresh-frozen ulnae in a repeated-measures model. Rounds of cyclic loading were applied at 1 Hz, for 100 cycles, increased in 50 N increments up to a maximum of 400 N. Micro-motion of the implant was quantified using an optical-tracking system. Outcome variables included total displacement, distal translation, gapping, anterior translation and axial stem rotation. Findings: Cement fixation reduced implant micro-motion compared to screw fixation, while the greatest implant micro-motion was observed in press-fit fixation. Comparing screw-fixation techniques, posterior–anterior screws provided superior stability only in distal translation. The implant did not experience displacements exceeding 0.9 mm with screw or cement fixation. Interpretation: Cement fixation provides the best initial fixation for a coronoid implant. However, the stability provided by both methods of screw fixation may be sufficient to allow osseous integration to be achieved for long-term fixation. Large displacements were observed using the press-fit fixation technique, suggesting that modifications would need to be developed and tested before this technique could be recommended for clinical application. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Traumatic elbow injuries can lead to significant elbow instability and disability. The coronoid process, which is a bony triangular eminence that projects from the anterior portion of the proximal ulna, is an integral component for maintaining elbow joint stability. This structure may be damaged in the setting of a traumatic injury, and given its importance for stability, Type II and larger coronoid fractures should be treated surgically with open reduction and internal fixation (ORIF) (Pollock et al., 2009a,b). While relatively uncommon, larger comminuted fractures that cannot be managed by ORIF pose a challenge in restoring elbow stability and can result in significant patient disability. The results of coronoid reconstruction with allografts and autografts have
⁎ Corresponding author at: 386 Yonge St. #1610, Toronto, ON M5B0A5, Canada. E-mail addresses: [email protected] (A.B. Gray), [email protected] (B. Alolabi), [email protected] (S. Deluce), [email protected] (L.M. Ferreira), [email protected] (G.S. Athwal), [email protected] (G.J.W. King), [email protected] (J.A. Johnson).
http://dx.doi.org/10.1016/j.clinbiomech.2015.11.017 0268-0033/© 2015 Elsevier Ltd. All rights reserved.
been historically poor, emphasizing the need for a reliable prosthetic device to adequately manage coronoid deficiencies (Chung et al., 2007; Van Riet et al., 2005). When ORIF is precluded due to comminution or osteopenia, a coronoid prosthesis may be a viable solution to restore elbow stability. Presently, there are no prosthetic devices of this nature available to address this clinical scenario. Biomechanical investigations have shown that initial stability of the implant is essential for proper fixation and osseous integration, which reduces the risk of component loosening and prolongs implant survival (Pilliar et al., 1986; Soballe et al., 1992). Excessive implant micro-motion has been shown to promote the ingrowth of fibrous tissue, which compared to bone ingrowth is insufficient to ensure long-term stability (Collier et al., 1988; Kienapfel et al., 1999; Pilliar et al., 1986; Soballe et al., 1992). The resulting aseptic loosening is the most common reason for performing revision surgeries of elbow prostheses, with revisions being typically more technically challenging, time consuming, costly, and having a higher incidence of complications than the initial joint replacement (Barrack, 1995; Barrack et al., 1995; Jafari et al., 2010; Kamineni & Morrey, 2004; Lavernia et al., 1995; Loebenberg et al., 2005; Ulrich et al., 2008).
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To our knowledge, no previous studies have examined the optimal fixation method for a prosthetic replacement for the coronoid process. Previous bio-mechanical investigations have shown that coronoid prostheses can restore stability to coronoid deficient cadaveric elbows. As such, examining potential fixation methods for implementation of such a device would be useful prior to clinical use. Press-fit cementless fixation, screw fixation, and cement fixation are all feasible options which should be investigated (Alolabi et al., 2011; Gray et al., 2013). Therefore, the purpose of this study was to determine the optimal fixation method of securing a coronoid prosthesis in the proximal ulna. We hypothesized that cement fixation would minimize the micro-motion of a coronoid prosthesis, while the greatest displacement would occur with the press-fit method. 2. Methods
ulna. The screw threads did not engage the prosthesis, thus achieving lag screw fixation. The AP screw holes were widened to accommodate larger machine screws (UNF#5-40, D = 3.18 mm) for PA fixation. These were passed anteriorly through the posterior surface of the ulna, through the prosthesis, and then were secured on its anterior surface with two stainless steel nuts. The screw threads did not engage the bone or the prosthesis. AP and PA screw fixation schematics are shown in Fig. 2. After the press fit and screw fixation techniques were tested, a rotary milling tool was used to widen the void in the metaphysis of the proximal ulna to create space for surgical cement (Simplex™ P bone cement, Stryker, Hamilton, ON, Canada). The opening in the fracture surface was then filled with cement, and the stem of the prosthesis was pressed into place and held stationary until the cement hardened. 2.3. Prosthesis tracking and loading protocol
2.1. Prosthesis design Morphological dimensions of the coronoid process from 18 cadaveric specimens (mean age 64.4 years [range 42 to 90 years], left arms: 11, right arms: 7, male arms: 11, female arms: 7) (Table 1) were measured from computed tomography scans (Mimics®: Materialise BV, Leuven, Belgium). A coronoid prosthesis was developed based on the following dimensions of the coronoid process: height, proximal-distal depth, medial-lateral width, and facet angles. Coronoid cartilage thickness was incorporated into the prosthesis design using the results from a parallel study conducted in our laboratory (Rafehi et al., 2011). The implant was fabricated from stainless steel using CAD–CAM technologies. Two threaded holes in the distal surface of the implant (Fig. 1) were used to attach and secure an optical tracking device, in order to monitor its motion relative to the ulna. Two through holes in the stem accommodate screws for antero-posterior (AP) and postero-anterior (PA) fixation methods. 2.2. Specimen preparation and mechanical test setup The soft tissues were removed from seven fresh frozen ulnae (mean age 79 years [range 60 to 88 years], left arms: 5, right arms: 2, male arms: 6, female arms: 1) (Table 1) and the distal sections of the bones were potted in bone cement. Digital calipers were used to measure the height of each coronoid process, and an oscillating sagittal saw was used to simulate a 40% (Type II) transverse coronoid fracture that is commonly encountered clinically (Doornberg et al., 2006; Regan & Morrey, 1989). The coronoid prosthesis was secured to the bone using the following four techniques in order: press-fit method, AP screws, PA screws, and cement. After the 40% coronoid osteotomy, a cavity slightly smaller than the implant stem was created in the cancellous bone located in the central region of the fractured surface. Press-fit fixation was accomplished by impacting the stem of the implant into the cavity. Two 2.4 mm cortical screws (Synthes, Missisauga, ON, Canada) were used for AP fixation. A 1.8 mm drill bit was used to drill holes directed posteriorly through the fracture surface, exiting through the posterior surface of the proximal ulna. The screws were then passed through the implant and securely fixed into the cortical bone of the posterior Table 1 Specimen data. The number, age, sex, and relative number of right and left specimens used for both the morphological (n = 18) and biomechanical study (n = 7).
Number of specimens (N) Left/right Male/female Mean age/range (years)
15
Morphological study
Biomechanical study
18 11/7 11/7 64 (42–90)
7 5/2 6/1 79 (60–88)
Optical trackers (Optotrak Certus®, NDI, Waterloo, ON, Canada) were mounted on the shaft of the ulna and on the prosthetic device to quantify motion of the prosthesis with respect to the ulna (Fig. 3). A stylus attached to a third optical tracker was used to digitize three points on the base of the articular face of the prosthesis (medial, ridge, and lateral) to monitor the micro-motion of the prosthesis throughout the testing protocol. The greater sigmoid notch and two flat points on the posterior surface of the ulna were also digitized in order to generate an anatomical coordinate system (shown in Fig. 4 with the three types of motion described in this study) for each specimen using a method previously described (McDonald et al., 2011; Sabo et al., 2011). Relative motions were determined with respect to the anatomical planes of the body. The four fixation methods were subjected to mechanical testing via a materials testing machine (Instron 8501®, Instron, Canton, MA, USA). Load was applied to the coronoid prosthesis using a custom designed fixture, developed to fit congruently against the medial and lateral facets of the implant. The load applicator was used to simulate loading imparted by the distal humerus. Distal loading was applied to the ridge of the implant's articular surface, midway between the base and tip of the prosthesis. Fixation methods were tested sequentially, first using the press-fit method, followed by AP, PA, and then cement fixation. Cyclic loading was applied to the prosthesis for eight cyclic loading regiments. Maximum cyclic loads ranged from 50 to 400 N in increments of 50 N, where each regiment lasted for 100 cycles at 1 Hz. Testing continued to the end of the loading protocol, or to the point of failure, which was defined as a displacement in any direction exceeding 2 mm at any one of the three digitized prosthesis points. The loading protocol used in this biomechanical study was based on the typical loads experienced within the human elbow. The maximum elbow flexion strength has been shown to occur at 90°, where a force of approximately three times the weight of the body has been estimated to pass through the elbow with heavy lifting (Kai-Nan & Morrey, 2000). Given this information, a force of approximately 2000 N may in some highly aggressive scenarios, be experienced in the elbow joint. The ulnohumeral joint is expected to experience 800 N of this force, based on the work of Halls and Travill, who reported that an applied axial force is distributed across the joint with 40% crossing the ulnohumeral joint and 60% crossing the radiohumeral articulation (Halls & Travill, 1964). This estimate of 800 N across the ulnohumeral joint is a maximum loading case; unlikely to be experienced by a patient postoperatively and would be distributed over the entire articular surface of the greater sigmoid notch. Applying a force of this magnitude to only the anterior 40% of the coronoid process would be well in excess of the regular physiological load experiences in this area, as such, the maximum load for this study was reduced to 400 N, approximately half of the maximum potential load, to more accurately represent potential loads experienced by patients in post-surgical rehabilitation,
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Fig. 1. The coronoid prosthesis. An isometric view of the coronoid prosthesis (a) used in the biomechanical testing is shown with anatomical orientation highlighted, where A indicates anterior, P indicates proximal, and M/L indicates medial or lateral. Two threaded holes in the distal surface of the prosthesis were used to attach and secure an optical tracking device to monitor its motion relative to the ulna, and two through holes through the stem accommodated antero-posterior (AP) and postero-anterior (PA) fixation methods. The stem used for press fit and cement fixation is highlighted in image d. Images b) c) and d) show further views of the implant from the distal aspect, bottom, and side profile respectively.
where extreme positions and excessive elbow stresses are avoided (Nirschl & Robert, 2000). Budoff et al. employed a similar loading protocol, where the coronoid was loaded posteriorly up to 445 N, in a study investigating screw and plate fixation methods for coronoid reconstruction (Budoff et al., 2011). 2.4. Outcome measures (dependent variables) and statistical analyses The total 3-dimensional displacement of the prosthesis was quantified for each fixation method to describe prosthesis micro-motion.
Three additional types of motion were analyzed to describe the behavior of the prosthesis with reference to the anatomical planes of the body; distal translation (Δd), gapping (Δg) (the anterior translation of the ridge point), and axial rotation (Δα) (rotation about the anterior– posterior axis). Distal translation was determined by tracking the displacement of the digitized ridge point parallel to the proximal–distal axis of the anatomical coordinate system, describing the motion of the prosthesis in the direction of the applied load. Articular gapping was measured by tracking the motion of the ridge point parallel to the anterior–posterior axis. The total displacement of the ridge point in 3-
Fig. 2. Coronoid prosthesis screw fixation. Anterior–posterior and posterior–anterior screw fixation methods are shown above. Two 2.4-mm cortical screws were used for anterior–posterior fixation as illustrated in (a) medial and (b) anterior views. Screws were passed through the body of the prosthesis and securely fastened into the cortical bone on the posterior surface of the ulna. For posterior–anterior fixation, two machine screws were passed through the bone and prosthesis, and secured on its anterior surface with two stainless steel nuts, illustrated in (c) medial and (d) anterior views.
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distal translation) throughout the duration of the loading protocol, categorized at the peak load levels (50 N, 100 N, 150 N, 200 N, 250 N, 300 N, 350 N, and 400 N). The press-fit fixation method was compared with the other methods throughout an abbreviated range of peak loads (50 N–150 N) as it did not consistently survive the entire testing protocol. 3. Results 3.1. Total displacement In a comparison of press-fit fixation with the other three fixation methods, the total displacement was found to be significantly less when the prosthesis was fixed with PA screws (P = 0.006), AP screws (P = 0.009), and cement (P = 0.004). Total displacement with cement fixation was found to be significantly less than the displacement for both PA and AP screws (P = 0.02). Total displacement was less with the PA verses the AP screws but this was not statistically different (P = 0.07) (Fig. 5a). 3.2. Distal translation
Fig. 3. Prosthesis motion with respect to coordinate axes. Two optical trackers were used to track prosthesis micro-motion relative to the stationary ulna, one secured to the coronoid prosthesis and another to the ulna. The loading direction is indicated by the vertical arrow, with the implant fixed with anterior–posterior screws.
dimensions, without reference to the anatomical coordinate system, was used as a failure criterion (N2 mm). Statistical comparisons between the fixation methods consisted of 2-way (peak load and fixation method) repeated-measures analysis of variance with significance defined at P b 0.05. Outcomes measures were the three different types of motion (axial rotation, gapping, and
The distal translation of the press-fit prosthesis was greater than both PA fixation (P = 0.001) and cement (P = 0.001). The distal translation of the press-fit method was greater than the AP fixation method, however this did not reach statistical significance (P = 0.09). Less distal translation was observed when the prosthesis was secured with cement fixation than either PA screw fixation (P = 0.04) or AP screw fixation (P = 0.01). The direction of screw fixation was also found to have a significant effect on the motion of the prosthesis in response to an applied load, where more translation was observed with AP screws in comparison to the PA screws (P = 0.04) (Fig. 5b). 3.3. Gapping displacement Significantly less prosthesis gapping was observed when the prosthesis was fixed with PA screws (P = 0.006), AP screws (P = 0.009), and cement (P = 0.006) than when press-fit fixation was employed. There was no difference in prosthesis gapping between the two screw fixation methods or cement fixation (P N 0.06) (Fig. 5c). 3.4. Axial rotation No differences were detected when the axial rotation of the prosthesis in press-fit fixation was compared to PA or AP fixation (P = 0.07 and P = 0.4 respectively). When cement was used to fix the prosthesis, less rotational motion was observed in comparison to the press-fit method (P = 0.02). Less rotation was observed when the prosthesis was secured with cement fixation as compared to both PA screw fixation (P = 0.03) and AP screw fixation (P = 0.02). No significant difference in rotational motion was detected between the two methods of screw fixation (P = 0.6) (Fig. 5d). 4. Discussion
Fig. 4. Prosthesis motion with respect to coordinate axes. In addition to total prosthesis displacement, three supplemental types of motion, which described prosthesis displacement and rotation with respect to the anatomical planes of the body, were measured. Distal translation (Δd) described motion of the prosthesis parallel to the proximal–distal axis. Gapping (Δg) described motion of the prosthesis parallel to the anterior–posterior axis; a displacement in this direction indicated a separation of the base of the prosthesis from the simulated fracture plane, causing a gap in the articular surfaces. Axial rotation (Δα) described the rotational motion of the prosthesis about the anterior–posterior axis.
As loosening is the most common cause for revision following elbow joint replacement surgery, establishing the optimal method for coronoid prosthesis fixation is useful clinically (Kamineni & Morrey, 2004; Loebenberg et al., 2005; Ulrich et al., 2008). As there are currently no commercially available prosthetic devices for the coronoid process, there is little available in the literature for comparison purposes. Previous biomechanical studies have reported reduced micro-motion and increased initial fixation strength with cement fixation (Bicknell et al., 2003; Jazrawi et al., 2001; Zheng et al., 2009). Our results generally agree with these findings, as cement fixation reduced prosthesis micro-motion when distal translation, axial rotation, and total motion
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Fig. 5. Prosthesis micro-motion in response to cyclic loading. The mean (+1 SD) displacement or rotation for the four fixation methods in response to cyclic loading for (a) total prosthesis displacement, (b), distal translation, (c) gapping displacement, and (d) axial rotation. The press-fit fixation method frequently reached only 150 N of the loading protocol, while cement and screw fixations consistently reached 400 N. The greatest prosthesis micro-motion was observed with press-fit fixation (P b 0.02), however no significant differences were found in axial rotation when press-fit fixation was compared to both types of screw fixation (P N 0.4), or when press-fit fixation was compared to anterior–posterior screws in distal translation (P N 0.09). Cement fixation largely minimized prosthesis micro-motion in comparison to both types of screw fixations (P b 0.03), with the exception of gapping displacement, where screw and cement fixation methods were not found to differ significantly (P N 0.3). Posterior–anterior screw fixation was found to provide superior stability in distal translation (P b 0.04) compared to anterior–posterior, however no significant differences were observed in the other three types of prosthesis motion (P N 0.05). The prosthesis did not experience displacements exceeding 0.9 mm in screw and cement fixations during the loading protocol.
were examined. Gapping motion was found to be quite minimal, and was similar in cement and screw fixation, not exceeding 0.2 mm. These small displacements reflect positively on both screw and cement fixation, suggesting only a small degree of separation is experienced between the articular surfaces of the prosthesis and coronoid, even at larger load magnitudes (400 N). The rotational and translational motion of the prosthesis was found to be quite minimal in general (less than 1° and 0.6 mm, respectively), reflecting positively on the future clinical application of a coronoid prosthesis, as it appears as though the original position of the prosthesis can be sufficiently maintained. Based on the results of this study, cement fixation of a coronoid prosthesis should provide secure initial fixation, particularly in older patients where uncemented fixation may be less successful due to poor bone quality. The results of this study suggest substantial micro-motion of the prosthesis can be expected if the prosthesis is fixed using a press-fit method. Furthermore, press-fit fixation could not consistently survive the loading protocol, further highlighting the inferior nature of this method. Collectively, this data suggests that press-fit fixation, using the prototype prosthesis in elderly cadavers, is not optimal for reducing micromotion, and would likely not provide sufficient stabilization required for osseous ingrowth (Kienapfel et al., 1999; Pilliar et al., 1986). In comparing the two screw fixation methods, the only significant difference noted was in distal translation, where PA screw orientation resulted in significantly less micro-motion. Overall, the magnitudes of the differences in micro-motion between screws were relatively small (b0.3 mm for distal translation), and may not be relevant clinically. The reliable nature of the PA screws, which does not depend on bone quality, and the ease of clinical application suggest this may be the preferred fixation option. Weaknesses of this study include the design of the prosthetic device, which consisted of a smooth press-fit stem, and did not include special
reamers and an oversized roughened stem. The stem design in this study was based on the anthropometric measurements of the 18 ulnae included in this study, further measurements from a larger sample population would be required to design a series of stem sizes appropriate for clinical use. We would also point out that the results of this study do not directly predict the response of long-term loading and in-vivo performance in response to repeated loading. While the relative strengths of the four fixation methods were quantified in this biomechanical study, and clinically significant loads were received by the implant with minimal micro-motion in response, the loading model used may not represent clinical failure. Furthermore, additional cycles, in excess of the 100 per loading step, may have been beneficial, as lower failure loads may have been observed. Although a small sample size was used in this study, the clear statistically significant findings indicate it was sufficient. A larger sample size may be useful however, to further differentiate between the relative strength of the two screw fixation methods. With respect to the modeling of load direction, when the elbow is flexed at 90°, the resulting load vector within the greater sigmoid notch has been shown to point somewhat posteriorly (Bicknell et al., 2003). In our model, the prosthesis was loaded in a direction parallel to the flat spot of the ulna. Although our model does not replicate the loading experienced physiologically in the elbow joint, it represents a worst-case scenario of prosthesis loading. Other limitations include the use of isolated denuded ulnae in the place of an intact joint. It is however likely that our results overestimated, rather than under-estimated, the magnitude of micromotion of the prosthesis, as the musculature and ligamentous structures of the elbow have been shown to contribute stabilizing effects to the joint (Fornalski et al., 2003). Second, it was not possible to randomize the order of the methods of prosthesis fixation. The effects of previous fixation could have been avoided by using separate cadaveric specimens
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for each method, however because of our preference to control for bone quality, fixation was performed repeatedly within the same specimen. As a result, the strength of the fixation methods performed later in the testing protocol may have been under-estimated, as the bone may have been weakened throughout testing. The repeated measures study design can also be considered a strength of this study, as it ensured the bone density was consistent for each fixation method. This study focuses on determining both the relative strengths of the fixation methods examined, as well as whether clinically significant loads can be withstood. Future studies should be conducted with a single fixation method per bone to more accurately determine accepted and failure loads. Strengths of this study include the quantification of the total 3dimensional motion of the prosthesis as well as specific types of motion using a state of the art optical tracking system. With this method we were able to isolate the displacement of the prosthesis with respect to the stationary ulna, unlike typical studies of this nature measuring crosshead displacement, which confound deformation of the bone and loading system. The cyclic loading protocol used, rather than constant loading, may be considered an advantage of this study, as it is a more clinically relevant method of load application, and thus more representative of the loading conditions experienced in the human body. Furthermore, a worst-case scenario was represented in this biomechanical study. The mean age of the cadaveric specimens at age 76 (range 60–84) represents a poorer bone quality than would be expected in the typical fracture patient, as such the strength of the fixation methods in these specimens was likely under-estimated. Had cadaveric specimens with higher bone quality been used, it is possible that even greater forces may have been accepted by each fixation method. The loading direction and magnitude chosen in this biomechanical study, where 400 N was applied to the tip of the coronoid, also likely overestimates the amount of force which would be physiologically experienced in a post operative patient. Although the use of older cadaver specimens does not represent the typical fracture patient, the use of poorer bone quality and high forces as described across the elbow joint results in a worstcase scenario. This may be considered a strength of the study, as it ensures the fixation methods were not over-estimated and suggests that both cement and screw fixation methods can truly withstand clinically relevant forces. The novel nature of this study, where the strengths of different fixation methods are compared may be considered both a strength, and a weakness, as at present, there are no similar gold standard studies in existence for comparison. 5. Conclusion In conclusion, micro-motion of a coronoid prosthesis was minimized when the prosthesis was fixed using cement in comparison to the other fixation techniques. Only small deviations from the original position of the prosthesis were observed with screw fixation, suggesting screw fixation could be a reasonable method for initial implant fixation. Large translational displacements were observed using the press-fit fixation technique employed in this study, suggesting modifications would need to be developed and tested before this could be recommended for clinical application. Acknowledgments We would like to acknowledge CIHR (The Canadian Institute of Health Research Operating Grant MOP- 106500) for funding contributions.
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