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Tendon lengthening after achilles tendon rupture–passive effects on the ankle joint in a cadaveric pilot study simulating weight bearing
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Tendon Lengthening After Achilles Tendon Rupture – Passive Effects On The Ankle Joint In A Cadaveric Pilot Study Simulating Weight Bearing Patrick M. Williamson BSc. , Jan PH. Pennings , Ethan Harlow M.D , Philip Hanna M.D , Aron Lechtig M.D , Stephen Okajima MS , Peter Biggane , Michael Nasr M.D. , David Zurakowski MSPhD , Naven Duggal M.D , Ara Nazarian Ph.D PII: DOI: Reference:
S0020-1383(19)30606-0 https://doi.org/10.1016/j.injury.2019.10.024 JINJ 8366
To appear in:
Injury
Accepted date:
12 October 2019
Please cite this article as: Patrick M. Williamson BSc. , Jan PH. Pennings , Ethan Harlow M.D , Philip Hanna M.D , Aron Lechtig M.D , Stephen Okajima MS , Peter Biggane , Michael Nasr M.D. , David Zurakowski MSPhD , Naven Duggal M.D , Ara Nazarian Ph.D , Tendon Lengthening After Achilles Tendon Rupture – Passive Effects On The Ankle Joint In A Cadaveric Pilot Study Simulating Weight Bearing, Injury (2019), doi: https://doi.org/10.1016/j.injury.2019.10.024
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Highlights
There is no consensus on the optimal treatment for Achilles tendon ruptures. Achilles tendon lengthening after conservative treatment may alter ankle kinetics. This study uses a cadaveric model that simulates static weight bearing to explore the effect of a lengthened Achilles tendon on ankle joint load distribution. Achilles lengthening does not significantly change contact pressures of the ankle joint in this model. Further studies are needed to address the effects of Achilles tendon lengthening on the ankle and elucidate the long-term clinical outcomes.
Tendon Lengthening After Achilles Tendon Rupture – Passive Effects On The Ankle Joint In A Cadaveric Pilot Study Simulating Weight Bearing Authors’ list: a) b) c) d) e) f) g) h) i) j) k)
Patrick, M. Williamson BSc. 1,2 Jan, PH. Pennings. 2 Ethan Harlow, M.D.3 Philip Hanna, M.D. 2 Aron Lechtig, M.D. 2 Stephen Okajima, MS. 2 Peter Biggane. 2 Michael Nasr, M.D. 2 David Zurakowski, MS, PhD 5 Naven Duggal, M.D. 4 Ara Nazarian, Ph.D. 2,6
Affiliations: 1) Boston University, Mechanical Engineering Department. Boston, Massachusetts 2) Center for Advanced Orthopaedic Studies at Beth Israel Deaconess Medical a. Center, Harvard Medical School. Boston, Massachusetts 3) Department of Orthopedic Surgery, University Hospitals Cleveland Medical Center. Cleveland, OH 4) Syracuse Orthopaedic Specialists, Department of General Orthopedics and Trauma, Foot and Ankle Division. Syracuse, NY 5) Department of Anesthesiology, Critical Care and Pain Medicine, Boston Children's Hospital, Harvard Medical School 6) Department of Orthopaedic Surgery, Yerevan State Medical University. Yerevan, Armenia Institutional address: a) Department of Orthopaedic Surgery at Beth Israel Deaconess Medical Center. 330 Brookline Avenue. Boston, MA. USA. 02115 Funding: a) Financial support by a grant from the American Orthopaedic Foot & Ankle Society with funding from the Orthopaedic Foot & Ankle Foundation was received and supported this study Correspondence: a) Corresponding Author: Ara Nazarian. Ph.D. b) Email: [email protected]
c) Center for Advanced Orthopaedic Studies at Beth Israel Deaconess Medical Center, Harvard Medical School. 99 Brookline Avenue, office RN-123a. Boston, MA. USA. 02115.
ABSTRACT Background: In recent years, primary Achilles tendon ruptures have increased due to the aging population’s participation in physically demanding activities. These injuries commonly occur during recreational sports and frequently lead to a long-term reduction in activity despite treatment. Non-operative methods of treatment for Achilles tendon ruptures may result in the Achilles healing in a lengthened position compared to the pre-injury state. This study uses a cadaveric model that simulates static weight bearing to explore the effect of a lengthened Achilles tendon on ankle joint load distribution. Methods: Five lower limb cadaveric specimens were placed on a custom jig, where a 334 N (75 lb) load was applied at the femoral head, and the foot was supported against a plate to simulate static double-leg stance. A pressure mapping sensor was inserted into the ankle joint. A percutaneous triple hemiresection tendo-Achilles lengthening procedure (Hoke procedure) was performed on each specimen to simulate tendon lengthening after conservative treatment. Contact pressure, peak pressure, and center-of-pressure were measured for native and tendon-lengthened conditions. Results: Tendon rupture did not significantly alter average contact pressure, peak contact pressures, or center-of-pressure in the ankle joint compared with native tendon. Conclusion: Achilles lengthening does not significantly change contact pressures of the ankle joint in this model . This result suggests that the passive restraint on ankle joint translation imposed by the Achilles tendon is minimal without muscle activation.
LEVEL OF EVIDENCE V KEYWORDS, Tendon Lengthening, Achilles Tendon, Cadaver, Cadaveric, Lower Leg, Kinetics INTRODUCTION As the largest tendon in the human body, the Achilles tendon conjoins the gastrocnemius and soleus muscles with the calcaneus and is integral to knee flexion, foot plantar flexion, and hindfoot inversion. [1, 2] Due to its involvement in many lower leg functions, the Achilles tendon is commonly injured with a prevalence as high as 55 per 100 thousand lower leg injuries [3]. Furthermore, Achilles injury is one of the four most common lower extremity musculoskeletal injuries subject to re-injury [4] and is missed as often as 25 percent of the time [58]. High eccentric contraction of the gastrocnemicus-soleus during extreme ankle dorsiflexion can cause Achilles tendon failure [9, 10]. The majority of Achilles tendon ruptures are related to recreational sports activities usually involving abrupt repetitive jumping and sprinting movements, or to an increase in activity level [11, Kvist, #127]. The highest incidence of these injuries are observed in 40- to 59-year-old men and women, though it occurs more frequently in men (80% of all acute Achilles tendon tears) [3]. Currently, there is no consensus whether Achilles tendon ruptures should be treated either conservatively or surgically [12, 13], but the healing process is often lengthy for both {Mazzone, #225}. The healing process of Achilles tendon rupture leads to unsatisfactory subjective and functional results, regardless of the type of treatment [13-15], though significantly worse functional results are recorded in conservative treatment [16-19]. Regarding the non-surgical treatment
in particular, complications include risk of re-rupture and pathologic elongation [20-25]. Elongation may be explained by scar tissue that fills the gap between two tendon ends [26-29]. The resulting lengthening causes cross-sectional area change, fibril reorganization, and collagen type change, which result in stiffness, reduced strength, and viscoelastic behavior change. [30-33] Together, these changes may alter the function of the ankle. With a lengthened Achilles tendon, the gastrocnemius’ and soleus’ ability to perform their functions may be impaired. This may imply a change in the kinetics of the ankle. Many attempts to understand the compensatory mechanisms after Achilles tendon rupture have yielded interesting observations like; side-to-side deficits in plantar function [39] and cavus deformity with anterior impingement in the ankle [40]. Importantly, a cavovarus foot deformity may be a long-term etiological factor for anteromedial ankle osteoarthrosis due to increased pressure and anteromedial shift of the center of force in the ankle joint [41]. Therefore, the minimization of Achilles tendon lengthening may be a determinant for the long-term recovery of ankle biomechanics [42]. Previous cadaveric studies that performed tendon lengthening have largely been limited to anatomic and surgical studies that describe and characterize procedures [44-47]. The Achilles tendon’s effect on ankle kinetics has not been explicitly explored and may serve as a foundation for understanding the compensatory mechanisms seen in Achilles lengthening following tendon rupture. In this study, we aim to preliminarily characterize the Achilles tendon’s passive restraint about the ankle joint with the foot in a neutral, or midstance, position subjected to physiologic weight bearing. We hypothesized that
lengthening the Achilles tendon would medialize the center-of-pressure supported by the talus. Additionally, we hypothesized that a lengthened Achilles would increase contact pressure experienced in the anterior half of the ankle joint.
MATERIALS AND METHODS Five fresh-frozen lower limb cadaveric specimens without known skeletal conditions were acquired from Medcure Anatomical Tissue Bank (Portland, OR, USA). The specifics of sample preparation have been thoroughly described previously [50, 51]. Briefly, a pressure mapping sensor (model 6900, TekScan, Boston, MA, USA) was secured within the ankle joint after thawing. The positioning of the sensor is shown in Figure 1. To accurately place the sensor, an anterior 4 cm longitudinal incision and an arthrotomy were performed to access the joint space. The pressure mapping sensor was placed inside the ankle joint to cover the articular surfaces of the distal tibia and talar dome. The sensor was further secured with sutures, as described previously [50, 51]. After placing the sensor, the femoral head was potted with Smooth Cast® Urethane Series 300 potting material (Smooth-On Inc, Easton, PA, USA) for positioning into the loading apparatus. The supine lower extremity with the foot in neutral position was loaded to 334 N (75 lbs) craniocaudally [51, 52].
The
loading apparatus was comprised of a steel screw-driven jack and a load cell (model LTH400 rated for 1000 lb, Futek, Irvine, CA, USA) mounted on a custom metal box frame (Figure 2) that allowed for precise specimen compression. Specimens were tested in an unaltered control condition (baseline) and after performing a lengthening procedure of the Achilles tendon (A percutaneous triple hemiresection TAL procedure (Hoke procedure) [53]). To confirm proper
lengthening, the specimen was evaluated in a non weight-bearing position, and ankle dorsiflexion of 30º was confirmed. Throughout data acquisition, the ankle was kept in neutral position by the loading apparatus. Each condition was tested three times, and measurements were averaged for each specimen. The pressure metrics (contact area, contact pressure, and peak pressure) were compared between conditions, and the displacement of the center-of-pressure (COP) was calculated between conditions for each joint. The position of the COP was calculated as a spatial average of the pressure over the sensing area for both the baseline and tendon lengthening conditions. The displacement of the COP was calculated as the distance between positions, taking the anterior and posterior translations as positive. To characterize the regional patterns of intraarticular pressure, the sensing area was also split into posterior and anterior halves, and the same comparisons as above were made (Table 1). The normality of the data was assessed using the Shapiro-Wilk test and Wilcoxon-Signed rank tests were performed to compare between conditions. All statistical analyses were performed using IBM SPSS Statistics software version 21.0 (IBM, Armonk, NY). Two-tailed p<0.05 was considered statistically significant.
RESULTS Center-of-Pressure On average, the COP within the ankle translated anteriorly (1.2 ± 0.8) and medially (0.1 ± 0.8mm) after the simulated rupture procedure, though the standard error is high. Figure 1 shows a representative sample of the pressure distribution within the ankle and the position of the COP.
Pressure Metrics There was no statistical difference in the contact pressure, peak pressure, and contact area within the ankle joint between the baseline and simulated Achilles tendon lengthening specimens (p > 0.59) Figure 3. Pressure in Anterior and Posterior Compartments The pressure in the anterior half of the ankle was not altered due to a simulated Achilles tendon lengthening. No significant changes were found in contact area, contact pressure, or peak pressure between conditions for either the anterior or posterior halves of the ankle joint (p > 0.22) Table 1.
DISCUSSION Simulated Achilles tendon lengthening did not result in statistically significant medial displacement of the joint center of pressure. Furthermore, Achilles tendon lengthening did not have a significant effect on the contact pressure measured within the anterior half of the ankle joint in this static weightbearing model of the lower limb. Numerous studies have shown that tension on individual muscles influences contact pressure and COP in the ankle joint. Our study sought to preliminarily characterize
ankle pressures with an elongated Achilles tendon
using a full lower limb cadaver loaded at the femoral head with 334 N (75 lbs.) to simulate half-body weight [52]. Unlike previous studies, we did not amputate the specimens midtibially or apply extrinsic forces to muscles traversing the ankle joint. Rather, samples were axially-loaded in a static fashion, and the knee joint was left intact, preserving the muscular origins. Of note, a freshly lengthened Achilles tendon would not have been suitable for extrinsic loading, whether variable or static, as this would have further deformed the tendon. Thus, our
study conditions were positioned best to preliminarily characterize the Achilles tendon’s passive restraint about the ankle joint with the foot in a neutral, or midstance, position subjected to physiologic weight bearing. The present pilot study of five cadaveric specimens demonstrated that the location of the COP, total contact pressure, and peak pressure in the ankle joint were not significantly altered by simulated Achilles lengthening during double leg stance. These findings warrant further investigation under conditions that employ variable dorsiflexion angles and both static and active loading. Interestingly, the mean COP within the ankle joint was displaced anteriorly and medially with Achilles rupture. This can be explained, in part, by the restraint on anterior displacement and the deforming eversion moment mediated by an intact Achilles tendon. Similar to previously repoted results from cadaveric experiments, [54] high inter-specimen variance was detected among native ankles in this study, with peak pressures and contact areas ranging from 0.03 MPa to 1.08 MPa and 30.1 to 174.2 mm2, respectively. However, the native ankles in our study demonstrated greater contact pressure in the posterior half than the anterior half of the joint. This finding is unique to our study, and our loading method, which highlights the complexity of femoral force transmission through bony and softtissue structures to the ankle joint. After the Achilles tendon lengthening, the mean posterior contact pressure decreased from 231 kPa to 165 kPa but did not coincide with a significant compensatory change in the anterior contact pressure. This trend should be further evaluated in models using femoral loading, given that numerous studies have substantiated a pattern of increased contact pressure anteriorly and laterally in the joint [54, 57, 58].
This study has limitations inherent to most ex-vivo pilot studies of the ankle joint under static loading conditions. Some alteration of the physiology may have occurred when opening the joint to place the pressure sensors, though this
was necessary for direct measurement of pressure within the joint. The study conditions were simplified to simulate double leg stance which does not involve changes in muscle activation; therefore, it does not reflect the changes in joint loading during movement. Although more representative of weight-bearing, the inclusion of the thigh and knee may serve as a source of error resultant from variable force transmission. Consistent placement and fixture of the pressure sensor was challenging due to the curved nature of the articular surface of the ankle, thereby limiting variability was limited by establishing an insertion protocol and fixing the leads to the surrounding soft tissue. Finally, the Hoke procedure was used to simulate the complex biologic process of tendon lengthening after conservative treatment in-vivo. Although not ideal, this model should simulate the elongation of the musculotendinous unit complex, and mimic its resulted effect as passive restraint on the ankle joint. Conservatively treated Achilles tendon ruptures can lead to distinctive Achilles tendon lengthening, calf muscle atrophy and low patient satisfaction [37, 59]. It is the task of future research to address the question whether the consequences of complete Achilles tendon rupture, such as tendon lengthening, soleus muscle atrophy, or deficits of the ankle function, lead to long-term changes and functional limitations of the lower limb.
CONCLUSION This pilot study showed that the loading topography and center of pressure of the ankle joint is not significantly influenced by lengthened Achilles
tendon in a full lower-limb model of weight-bearing under static loading conditions. These findings warrant further investigation under conditions that employ dynamic motion and muscle loading. Conflict of Interest
We wish to confirm that there are no known conflicts of interest associated with this manuscript, and there has been no significant financial support for this work that could have influenced its outcome. ACKNOWLEDGEMENTS The authors would like to acknowledge the American Orthopaedic Foot and Ankle Society for supporting this project (N.D. and A.N.).
Funding The authors disclosed receipt of the following financial support for research, authorship, and/or publication of this article: Support by a grant from the American Orthopaedic Foot & Ankle Society with funding from the Orthopaedic Foot & Ankle Foundation.
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Figure 1. Illustrative image of the pressure-sensitive film placement on the articular surface of a right ankle. The red box indicates the center of pressure and the legend is in MPa (maximum 1.8 MPa in pink).
Figure 2: Diagram of the loading apparatus from above including the placement of the leg in supine position, screw-driven jack, load cell (gray box between scissor jack and femoral potting box), pressure sensor (red), and the adjustable platform (below the foot) that allowed for proper foot placement.
Figure 3. Comparison of Median Contact Pressure and Median Peak Pressure measured in MPa within the ankle joint at baseline and tendon lengthening conditions. Error bars show the 25th and 75th quartiles. Neither comparison of pressure metrics between loading conditions showed significance (Contact Pressure: p = 0.89 and Peak Pressure p = 0.89).
Tendon Lengthening After Achilles Tendon Rupture – Passive Effects On The Ankle Joint In A Cadaveric Pilot Study Simulating Weight Bearing Patrick M. Williamson BSc. , Jan PH. Pennings , Ethan Harlow M.D , Philip Hanna M.D , Aron Lechtig M.D , Stephen Okajima MS , Peter Biggane , Michael Nasr M.D. , David Zurakowski MSPhD , Naven Duggal M.D , Ara Nazarian Ph.D PII: DOI: Reference:
S0020-1383(19)30606-0 https://doi.org/10.1016/j.injury.2019.10.024 JINJ 8366
To appear in:
Injury
Accepted date:
12 October 2019
Please cite this article as: Patrick M. Williamson BSc. , Jan PH. Pennings , Ethan Harlow M.D , Philip Hanna M.D , Aron Lechtig M.D , Stephen Okajima MS , Peter Biggane , Michael Nasr M.D. , David Zurakowski MSPhD , Naven Duggal M.D , Ara Nazarian Ph.D , Tendon Lengthening After Achilles Tendon Rupture – Passive Effects On The Ankle Joint In A Cadaveric Pilot Study Simulating Weight Bearing, Injury (2019), doi: https://doi.org/10.1016/j.injury.2019.10.024
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Highlights
There is no consensus on the optimal treatment for Achilles tendon ruptures. Achilles tendon lengthening after conservative treatment may alter ankle kinetics. This study uses a cadaveric model that simulates static weight bearing to explore the effect of a lengthened Achilles tendon on ankle joint load distribution. Achilles lengthening does not significantly change contact pressures of the ankle joint in this model. Further studies are needed to address the effects of Achilles tendon lengthening on the ankle and elucidate the long-term clinical outcomes.
Tendon Lengthening After Achilles Tendon Rupture – Passive Effects On The Ankle Joint In A Cadaveric Pilot Study Simulating Weight Bearing Authors’ list: a) b) c) d) e) f) g) h) i) j) k)
Patrick, M. Williamson BSc. 1,2 Jan, PH. Pennings. 2 Ethan Harlow, M.D.3 Philip Hanna, M.D. 2 Aron Lechtig, M.D. 2 Stephen Okajima, MS. 2 Peter Biggane. 2 Michael Nasr, M.D. 2 David Zurakowski, MS, PhD 5 Naven Duggal, M.D. 4 Ara Nazarian, Ph.D. 2,6
Affiliations: 1) Boston University, Mechanical Engineering Department. Boston, Massachusetts 2) Center for Advanced Orthopaedic Studies at Beth Israel Deaconess Medical a. Center, Harvard Medical School. Boston, Massachusetts 3) Department of Orthopedic Surgery, University Hospitals Cleveland Medical Center. Cleveland, OH 4) Syracuse Orthopaedic Specialists, Department of General Orthopedics and Trauma, Foot and Ankle Division. Syracuse, NY 5) Department of Anesthesiology, Critical Care and Pain Medicine, Boston Children's Hospital, Harvard Medical School 6) Department of Orthopaedic Surgery, Yerevan State Medical University. Yerevan, Armenia Institutional address: a) Department of Orthopaedic Surgery at Beth Israel Deaconess Medical Center. 330 Brookline Avenue. Boston, MA. USA. 02115 Funding: a) Financial support by a grant from the American Orthopaedic Foot & Ankle Society with funding from the Orthopaedic Foot & Ankle Foundation was received and supported this study Correspondence: a) Corresponding Author: Ara Nazarian. Ph.D. b) Email: [email protected]
c) Center for Advanced Orthopaedic Studies at Beth Israel Deaconess Medical Center, Harvard Medical School. 99 Brookline Avenue, office RN-123a. Boston, MA. USA. 02115.
ABSTRACT Background: In recent years, primary Achilles tendon ruptures have increased due to the aging population’s participation in physically demanding activities. These injuries commonly occur during recreational sports and frequently lead to a long-term reduction in activity despite treatment. Non-operative methods of treatment for Achilles tendon ruptures may result in the Achilles healing in a lengthened position compared to the pre-injury state. This study uses a cadaveric model that simulates static weight bearing to explore the effect of a lengthened Achilles tendon on ankle joint load distribution. Methods: Five lower limb cadaveric specimens were placed on a custom jig, where a 334 N (75 lb) load was applied at the femoral head, and the foot was supported against a plate to simulate static double-leg stance. A pressure mapping sensor was inserted into the ankle joint. A percutaneous triple hemiresection tendo-Achilles lengthening procedure (Hoke procedure) was performed on each specimen to simulate tendon lengthening after conservative treatment. Contact pressure, peak pressure, and center-of-pressure were measured for native and tendon-lengthened conditions. Results: Tendon rupture did not significantly alter average contact pressure, peak contact pressures, or center-of-pressure in the ankle joint compared with native tendon. Conclusion: Achilles lengthening does not significantly change contact pressures of the ankle joint in this model . This result suggests that the passive restraint on ankle joint translation imposed by the Achilles tendon is minimal without muscle activation.
LEVEL OF EVIDENCE V KEYWORDS, Tendon Lengthening, Achilles Tendon, Cadaver, Cadaveric, Lower Leg, Kinetics INTRODUCTION As the largest tendon in the human body, the Achilles tendon conjoins the gastrocnemius and soleus muscles with the calcaneus and is integral to knee flexion, foot plantar flexion, and hindfoot inversion. [1, 2] Due to its involvement in many lower leg functions, the Achilles tendon is commonly injured with a prevalence as high as 55 per 100 thousand lower leg injuries [3]. Furthermore, Achilles injury is one of the four most common lower extremity musculoskeletal injuries subject to re-injury [4] and is missed as often as 25 percent of the time [58]. High eccentric contraction of the gastrocnemicus-soleus during extreme ankle dorsiflexion can cause Achilles tendon failure [9, 10]. The majority of Achilles tendon ruptures are related to recreational sports activities usually involving abrupt repetitive jumping and sprinting movements, or to an increase in activity level [11, Kvist, #127]. The highest incidence of these injuries are observed in 40- to 59-year-old men and women, though it occurs more frequently in men (80% of all acute Achilles tendon tears) [3]. Currently, there is no consensus whether Achilles tendon ruptures should be treated either conservatively or surgically [12, 13], but the healing process is often lengthy for both {Mazzone, #225}. The healing process of Achilles tendon rupture leads to unsatisfactory subjective and functional results, regardless of the type of treatment [13-15], though significantly worse functional results are recorded in conservative treatment [16-19]. Regarding the non-surgical treatment
in particular, complications include risk of re-rupture and pathologic elongation [20-25]. Elongation may be explained by scar tissue that fills the gap between two tendon ends [26-29]. The resulting lengthening causes cross-sectional area change, fibril reorganization, and collagen type change, which result in stiffness, reduced strength, and viscoelastic behavior change. [30-33] Together, these changes may alter the function of the ankle. With a lengthened Achilles tendon, the gastrocnemius’ and soleus’ ability to perform their functions may be impaired. This may imply a change in the kinetics of the ankle. Many attempts to understand the compensatory mechanisms after Achilles tendon rupture have yielded interesting observations like; side-to-side deficits in plantar function [39] and cavus deformity with anterior impingement in the ankle [40]. Importantly, a cavovarus foot deformity may be a long-term etiological factor for anteromedial ankle osteoarthrosis due to increased pressure and anteromedial shift of the center of force in the ankle joint [41]. Therefore, the minimization of Achilles tendon lengthening may be a determinant for the long-term recovery of ankle biomechanics [42]. Previous cadaveric studies that performed tendon lengthening have largely been limited to anatomic and surgical studies that describe and characterize procedures [44-47]. The Achilles tendon’s effect on ankle kinetics has not been explicitly explored and may serve as a foundation for understanding the compensatory mechanisms seen in Achilles lengthening following tendon rupture. In this study, we aim to preliminarily characterize the Achilles tendon’s passive restraint about the ankle joint with the foot in a neutral, or midstance, position subjected to physiologic weight bearing. We hypothesized that
lengthening the Achilles tendon would medialize the center-of-pressure supported by the talus. Additionally, we hypothesized that a lengthened Achilles would increase contact pressure experienced in the anterior half of the ankle joint.
MATERIALS AND METHODS Five fresh-frozen lower limb cadaveric specimens without known skeletal conditions were acquired from Medcure Anatomical Tissue Bank (Portland, OR, USA). The specifics of sample preparation have been thoroughly described previously [50, 51]. Briefly, a pressure mapping sensor (model 6900, TekScan, Boston, MA, USA) was secured within the ankle joint after thawing. The positioning of the sensor is shown in Figure 1. To accurately place the sensor, an anterior 4 cm longitudinal incision and an arthrotomy were performed to access the joint space. The pressure mapping sensor was placed inside the ankle joint to cover the articular surfaces of the distal tibia and talar dome. The sensor was further secured with sutures, as described previously [50, 51]. After placing the sensor, the femoral head was potted with Smooth Cast® Urethane Series 300 potting material (Smooth-On Inc, Easton, PA, USA) for positioning into the loading apparatus. The supine lower extremity with the foot in neutral position was loaded to 334 N (75 lbs) craniocaudally [51, 52].
The
loading apparatus was comprised of a steel screw-driven jack and a load cell (model LTH400 rated for 1000 lb, Futek, Irvine, CA, USA) mounted on a custom metal box frame (Figure 2) that allowed for precise specimen compression. Specimens were tested in an unaltered control condition (baseline) and after performing a lengthening procedure of the Achilles tendon (A percutaneous triple hemiresection TAL procedure (Hoke procedure) [53]). To confirm proper
lengthening, the specimen was evaluated in a non weight-bearing position, and ankle dorsiflexion of 30º was confirmed. Throughout data acquisition, the ankle was kept in neutral position by the loading apparatus. Each condition was tested three times, and measurements were averaged for each specimen. The pressure metrics (contact area, contact pressure, and peak pressure) were compared between conditions, and the displacement of the center-of-pressure (COP) was calculated between conditions for each joint. The position of the COP was calculated as a spatial average of the pressure over the sensing area for both the baseline and tendon lengthening conditions. The displacement of the COP was calculated as the distance between positions, taking the anterior and posterior translations as positive. To characterize the regional patterns of intraarticular pressure, the sensing area was also split into posterior and anterior halves, and the same comparisons as above were made (Table 1). The normality of the data was assessed using the Shapiro-Wilk test and Wilcoxon-Signed rank tests were performed to compare between conditions. All statistical analyses were performed using IBM SPSS Statistics software version 21.0 (IBM, Armonk, NY). Two-tailed p<0.05 was considered statistically significant.
RESULTS Center-of-Pressure On average, the COP within the ankle translated anteriorly (1.2 ± 0.8) and medially (0.1 ± 0.8mm) after the simulated rupture procedure, though the standard error is high. Figure 1 shows a representative sample of the pressure distribution within the ankle and the position of the COP.
Pressure Metrics There was no statistical difference in the contact pressure, peak pressure, and contact area within the ankle joint between the baseline and simulated Achilles tendon lengthening specimens (p > 0.59) Figure 3. Pressure in Anterior and Posterior Compartments The pressure in the anterior half of the ankle was not altered due to a simulated Achilles tendon lengthening. No significant changes were found in contact area, contact pressure, or peak pressure between conditions for either the anterior or posterior halves of the ankle joint (p > 0.22) Table 1.
DISCUSSION Simulated Achilles tendon lengthening did not result in statistically significant medial displacement of the joint center of pressure. Furthermore, Achilles tendon lengthening did not have a significant effect on the contact pressure measured within the anterior half of the ankle joint in this static weightbearing model of the lower limb. Numerous studies have shown that tension on individual muscles influences contact pressure and COP in the ankle joint. Our study sought to preliminarily characterize
ankle pressures with an elongated Achilles tendon
using a full lower limb cadaver loaded at the femoral head with 334 N (75 lbs.) to simulate half-body weight [52]. Unlike previous studies, we did not amputate the specimens midtibially or apply extrinsic forces to muscles traversing the ankle joint. Rather, samples were axially-loaded in a static fashion, and the knee joint was left intact, preserving the muscular origins. Of note, a freshly lengthened Achilles tendon would not have been suitable for extrinsic loading, whether variable or static, as this would have further deformed the tendon. Thus, our
study conditions were positioned best to preliminarily characterize the Achilles tendon’s passive restraint about the ankle joint with the foot in a neutral, or midstance, position subjected to physiologic weight bearing. The present pilot study of five cadaveric specimens demonstrated that the location of the COP, total contact pressure, and peak pressure in the ankle joint were not significantly altered by simulated Achilles lengthening during double leg stance. These findings warrant further investigation under conditions that employ variable dorsiflexion angles and both static and active loading. Interestingly, the mean COP within the ankle joint was displaced anteriorly and medially with Achilles rupture. This can be explained, in part, by the restraint on anterior displacement and the deforming eversion moment mediated by an intact Achilles tendon. Similar to previously repoted results from cadaveric experiments, [54] high inter-specimen variance was detected among native ankles in this study, with peak pressures and contact areas ranging from 0.03 MPa to 1.08 MPa and 30.1 to 174.2 mm2, respectively. However, the native ankles in our study demonstrated greater contact pressure in the posterior half than the anterior half of the joint. This finding is unique to our study, and our loading method, which highlights the complexity of femoral force transmission through bony and softtissue structures to the ankle joint. After the Achilles tendon lengthening, the mean posterior contact pressure decreased from 231 kPa to 165 kPa but did not coincide with a significant compensatory change in the anterior contact pressure. This trend should be further evaluated in models using femoral loading, given that numerous studies have substantiated a pattern of increased contact pressure anteriorly and laterally in the joint [54, 57, 58].
This study has limitations inherent to most ex-vivo pilot studies of the ankle joint under static loading conditions. Some alteration of the physiology may have occurred when opening the joint to place the pressure sensors, though this
was necessary for direct measurement of pressure within the joint. The study conditions were simplified to simulate double leg stance which does not involve changes in muscle activation; therefore, it does not reflect the changes in joint loading during movement. Although more representative of weight-bearing, the inclusion of the thigh and knee may serve as a source of error resultant from variable force transmission. Consistent placement and fixture of the pressure sensor was challenging due to the curved nature of the articular surface of the ankle, thereby limiting variability was limited by establishing an insertion protocol and fixing the leads to the surrounding soft tissue. Finally, the Hoke procedure was used to simulate the complex biologic process of tendon lengthening after conservative treatment in-vivo. Although not ideal, this model should simulate the elongation of the musculotendinous unit complex, and mimic its resulted effect as passive restraint on the ankle joint. Conservatively treated Achilles tendon ruptures can lead to distinctive Achilles tendon lengthening, calf muscle atrophy and low patient satisfaction [37, 59]. It is the task of future research to address the question whether the consequences of complete Achilles tendon rupture, such as tendon lengthening, soleus muscle atrophy, or deficits of the ankle function, lead to long-term changes and functional limitations of the lower limb.
CONCLUSION This pilot study showed that the loading topography and center of pressure of the ankle joint is not significantly influenced by lengthened Achilles
tendon in a full lower-limb model of weight-bearing under static loading conditions. These findings warrant further investigation under conditions that employ dynamic motion and muscle loading. Conflict of Interest
We wish to confirm that there are no known conflicts of interest associated with this manuscript, and there has been no significant financial support for this work that could have influenced its outcome. ACKNOWLEDGEMENTS The authors would like to acknowledge the American Orthopaedic Foot and Ankle Society for supporting this project (N.D. and A.N.).
Funding The authors disclosed receipt of the following financial support for research, authorship, and/or publication of this article: Support by a grant from the American Orthopaedic Foot & Ankle Society with funding from the Orthopaedic Foot & Ankle Foundation.
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Figure 1. Illustrative image of the pressure-sensitive film placement on the articular surface of a right ankle. The red box indicates the center of pressure and the legend is in MPa (maximum 1.8 MPa in pink).
Figure 2: Diagram of the loading apparatus from above including the placement of the leg in supine position, screw-driven jack, load cell (gray box between scissor jack and femoral potting box), pressure sensor (red), and the adjustable platform (below the foot) that allowed for proper foot placement.
Figure 3. Comparison of Median Contact Pressure and Median Peak Pressure measured in MPa within the ankle joint at baseline and tendon lengthening conditions. Error bars show the 25th and 75th quartiles. Neither comparison of pressure metrics between loading conditions showed significance (Contact Pressure: p = 0.89 and Peak Pressure p = 0.89).