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Sub-optimal location of locking screw positioning in tibio-talo-calcaneal arthrodesis with an intramedullary device
Foot and Ankle Surgery 13 (2007) 122–125 www.elsevier.com/locate/fas
Sub-optimal location of locking screw positioning in tibio-talo-calcaneal arthrodesis with an intramedullary device Andrew F. Young, Steven Gwilym, Paul Cooke *, Robert Sharp Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX3 7LD, United Kingdom Received 2 October 2006; accepted 29 January 2007
Abstract Background: The intramedullary ankle nail for tibio-talo-calcaneal (TCC) fusion has been noted in clinical practice to have locking screw holes that do not correspond to the ideal screw placement for bony purchase. Methods: Retrospective review identified 23 patients who had undergone 25 TCC arthrodeses. A retrospective study of the patient radiographs and records were undertaken. Results: The distal PA locking screw hole is significantly too high up the nail itself to facilitate placement of the screw in the optimal position within the calcaneum. The inferior hole should be placed lower and the superior hole placed higher to allow placement of the screws within the calcaneus and talus, respectively. Conclusions: Future designs of intramedullary nailing systems may consider our findings when deciding on screw positions to facilitate the attainment of a solid, pain-free arthrodesis. Future study is required into different designs with different screw options for different operative indications. # 2007 European Foot and Ankle Society. Published by Elsevier Ltd. All rights reserved. Keywords: Ankle; Arthrodesis; Fusion; Intramedullary device
1. Introduction The indications for tibiotalocalcaneal fusion include osteo or inflammatory arthritides affecting ankle and subtalar joint, large deformities of the hind-foot complex and salvage following failed total ankle replacement. It can be associated with high patient satisfaction but the procedure is demanding and complication rates can be significant [1]. It is often preferable to amputation however [2]. The option of intramedullary fixation for tibiotalocalcaneal (TCC) fusion was first described in 1948 by Adams and the exact form has taken many guises since [3]. A longer rod configuration than those tested here is advocated for use in the presence of systemic or localised osteopenia [4]. The ankle arthrodesis nail is a tibiotalocalcaneal reamed fusion device designed for the relief of pain and deformity by means of fusion [5]. It is a second generation nail that * Corresponding author. Tel.: +44 1865 741155; fax: +44 1865 742348. E-mail address: [email protected] (P. Cooke).
affords rigid, load sharing fixation and is available in 15 and 18 cm lengths and varied diameters. The process of tibiotalocalcaneal fusion involves ankle and subtalar arthrotomies through a lateral fibular excision, preparation of the joint surfaces, and then placement of the nail through a plantar incision. Screws are placed proximally into the tibia and, after compression utilising a distal compression device [6], the nail can be locked distally with two lateral screws into the calcaneus and talus with the option of an additional posterior–anterior calcaneal locking screw. The goal is the production of a solid, pain-free arthrodesis in a biomechanically stable and functional position [6]. This study is concerned with the ankle arthrodesis nail and the placement of the screw holes for the lateral–medial (LM) and posterior–anterior (PA) distal locking screws. These had been observed by the senior authors to be suboptimally placed in some cases, especially in patients where loss of normal anatomy from destructive processes such as rheumatoid arthritis or failing arthroplasty had occurred. The hypothesis is that the distal PA locking and distal
1268-7731/$ – see front matter # 2007 European Foot and Ankle Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.fas.2007.01.005
A.F. Young et al. / Foot and Ankle Surgery 13 (2007) 122–125
123
(inferior) LM screw holes are too high (superior), while the proximal (superior) LM screw hole is too low (inferior). A retrospective study of position of screw placement was performed in patients who had undergone TTC fusion.
2. Methods Thirty patients who had undergone tibiotalocalcaneal arthrodesis were identified from computerised theatre records. These procedures were all carried out between June 2003 and March 2004. These patients were operated on by two consultant foot and ankle orthopaedic surgeons. The patient’s records and radiographs were retrospectively reviewed. Twenty-three patients were identified who underwent 25 tibiotalocalcaneal fusions with the Biomet arthrodesis nail. Seven patients were excluded as they had undergone additional procedures. The radiographs were reviewed and the post procedure films were measured to identify the heights of the ankle and subtalar joints (see Fig. 1). This was performed on the lateral films. The patient records were used to identify the length of nail (150 or 180 mm) implant used. The measurements were then calibrated to disregard the magnification (in the region of 15%) inherent to standard film radiographs as the true length of nail was known. The measurements could thus be standardised to give a true reading of the heights of the ankle and subtalar joints at the centre of the bony ankle; the site of the nail implant. This allowed the radiographs to be standardised and thus comparable for the purposes of the study. The heights of the top and bottom of the ankle and
Fig. 1. The points measured on the radiographs. The vertical line simulates the line of insertion of the arthrodesis nail.
Fig. 2. The ideal screw hole placements. The circles represent the ideal screw holes in an arthrodesis nail.
subtalar joints were measured (represented by points A and B, and C and D, respectively on the figure) to correctly identify the centre of the joints. The protrusion of the implant inferiorly from the calcaneus was also measured and recorded (point E). Through this method the calcaneus, talus and ankle and sub-talar joints could be mapped out mathematically. The AP radiographs were ignored for the purposes of this study as they added no further information (Figs. 2 and 3). We assumed the ideal place for placement of a single screw would be the centre of the bone, and the ideal placement for two screws would be one third and two thirds up the height of the bone. This allows for the greatest margin of error to account for normal variation in the human
Fig. 3. Graphical representation of the ideal screw hole placements. The mean is taken to be the ideal placement. Highest to lowest represents the range of the vertical height.
124
A.F. Young et al. / Foot and Ankle Surgery 13 (2007) 122–125
population. The measurements could then be used to give these ideal placements for each patient. These were then compared to the screw hole placement along the ankle arthrodesis nail (15.5, 25.5, and 40 mm from the distal end). The patient sample was divided into two groups to aid analysis; those whose operative indication was primary osteoarthritis and those whose indication was any other. This was designed to exclude any possible bias introduced by the loss of joint and bone height associated with previous operative intervention. In particular, salvage of failed total ankle replacement is hampered by extensive bone loss, osteopenia, and hind-foot pathology [7]. A related t-test was used to determine if any observed differences were significant ( p < 0.05). The surgical indication for the procedure was identified and recorded.
3. Results The study patients underwent tibiotalocalcaneal fusion for arthritis (16 patients), failure of total ankle replacement (5), failed screw ankle fusion (2), or after fracture with severe arthritis (2). The nails were 150 mm (11) or 180 mm (14) in length. The surgical procedures were carried out by two consultant orthopaedic surgeons. The distal hole for the posterior to anterior (PA) distal screw is significantly too high up the nail to facilitate placement within the calcaneus (mean 11.36 mm, range 7.3– 16.1, p < 0.001). The lateral–medial (LM) calcaneal and talar locking screw holes should be placed lower (mean 22.73, range 14.5–32.3, p < 0.01) and higher (mean 43.74, range 23.0–57.9, p < 0.05), respectively to facilitate placement of one within the talus and one within the calcaneus. When the results are divided into the ‘arthritis’ and ‘other’ groups by operative indication then the significance of the differences changes. This may suggest that the screw holes may need to be different according to operative indication. This is displayed in Table 2. The findings suggest that the intramedullary device for arthritis would have screw holes further from the end than a similar device for different operative indications. The table also reveals that only in the case of the LM calcaneal screw in the arthritis group and the LM talar screw in the other group are the present screw holes not significantly different.
4. Discussion Tibio-talo-calcaneal fusion is a technically demanding procedure with high rates of complications. These include high non-union rates [8,9] and wound infection [10]. The nail has evolved over time from a configuration with transverse locking screws only to the current second generation design with a PA screw replacing one of the
Table 1 Mean joint heights (mm)
Tibio-talar joint Sub-talar joint
All subjects
Arthritis group
Other group
53.4 34.1
54.3 35.1
51.8 32.2
Height in millimetres of joints from bottom of calcaneus at point of entry or arthrodesis nail.
transverse screws in the traditional intramedullary nail configuration. This was due to problems of placing a locking screw consistently from the lateral calcaneal body through the rod and into the sustenaculum tali [8]. The distal hole for the posterior to anterior (PA) distal screw is significantly too high up the nail to facilitate placement within the calcaneus (mean 11.36 mm, range 7.3– 16.1, p < 0.001). The lateral–medial (LM) calcaneal and talar locking screw holes should be placed lower (mean 22.73, range 14.5–32.3, p < 0.01) and higher (mean 43.74, range 23.0–57.9, p < 0.05), respectively to facilitate placement of one within the talus and one within the calcaneus. The mean joint heights shown in Table 1 are slightly different to those found in a study Hepple et al. [11] but those measurements had been taken from a normal population and measured in a standardised pattern, whereas the study here is concerned with the heights of the joints in line with the line of introduction of the arthrodesis nail. The measurements in the study undertaken by Hepple et al. would support this study (Table 2). The findings when the results were divided by operative indication would suggest that an intramedullary device designed specifically for patients with severe ankle osteoarthritis would have screw holes at slightly different positions to those for different operative indications. These findings were not significantly different in this study and thus would not warrant separate designs. This should be subjected to further testing. The PA locking screw passes through the calcaneus in a longitudinal manner, providing good bone purchase, and can engage stronger bone such as the sustenaculum tali. In this study, this corresponds to the hypothesised ideal PA screw placement. The placement of a posterior–anterior locking screw has been shown to be significantly stiffer than a lateral–medial screw 8 and thus, we can assume, of great importance. The PA approach affords better purchase in bone, when compared to the LM construct, and helps to neutralize sagittal forces at the fusion site, and thus may help Table 2 Ideal screw hole placements
LM talar screw LM calcaneal screw PA calcaneal screw a
All patients
Arthritis group
Other group
Actual hole
43.7a 22.7a 11.4a
44.7a 23.4 11.7a
42.0 21.5a 10.7a
40 mm 25.5 mm 15.5 mm
Significantly different to actual screw hole height ( p < 0.05).
A.F. Young et al. / Foot and Ankle Surgery 13 (2007) 122–125
to improve fusion rates [3]. The current study has shown that this PA screw hole is sub-optimally placed and should thus be altered in the design of the future ankle arthrodesis nails. The LM screw holes have been shown by this study to be in positions that do not allow the transverse screws to be placed within the centre of the talus or equidistant between the PA screw and the top of the calcaneusm. The position of screw holes in future TCC nail designs should allow for the findings of this study to ensure optimum positioning.
[4]
[5] [6] [7] [8]
References [9] [1] Hammett R, Hepple S, Forster B, Harries W, Winson I. Tibio-talocalcaneal (hind-foot) arthrodesis by intramedullary nailing. Results of 54 cases. Abstract. J Bone Joint Surg 2003;85:246. [2] Miller S. Techniques and prevention of complications for intramedullary nailing arthrodesis of the ankle. Tech Ortho 2001;16:372–80. [3] Mader K, Pennig D, Gausepohl T, Patsalis T. Calcaneotalotibial arthrodesis with a retrograde posterior-to-anterior locked nail as a
[10] [11]
125
salvage procedure for severe ankle pathology. J Bone Joint Surg 2003;85-A(4):123–8. Noonan T, Pinzur M, Paxinos O, Havey R, Patwardhin A. Tibiotalocalcaneal arthrodesis with a retrograde intramedullary nail: a biomechanical analysis of the effect of nail length. Foot Ankle Int 2005;26:304–8. Biomet Inc. Surgical Technique Guide Ankle Arthrodesis Nail. Biomet Inc.; 2005. Quill G. Tibiotalocalcaneal arthrodesis with medullary rod fixation. Tech Foot Ankle Surg 2003;2:135–43. Bruggeman N, Kitaoka H. Arthrodesis after failed total ankle arthroplasty. Tech Foot Ankle Surg 2002;1:60–8. Mann MR, Parks BG, Pak SS, Miller SD. Tibiotalocalcaneal arthrodesis: a biomechanical analysis of the rotational stability of the Biomet ankle arthrodesis nail. Foot Ankle Int 2001;22:731–3. Frey C, Halikus NM, Vu-Rose T, Ebramzadeh E. A review of ankle arthrodesis: predisposing factors to non-union. Foot Ankle Int 1994;15:581–4. Chou L, Mann R, Yaszay B, Graves S, McPeake W, Dreeban S, et al. Tibiotalocalcaneal arthrodesis. Foot Ankle Int 2000;21:804–8. Hepple S, Doling N, Winson I, Glew D. Hind-foot alignment: normal values and their relevance to the tibial-calcaneal arthrodesis. Abstract. J Bone Joint Surg 1999;81:294.
Sub-optimal location of locking screw positioning in tibio-talo-calcaneal arthrodesis with an intramedullary device Andrew F. Young, Steven Gwilym, Paul Cooke *, Robert Sharp Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX3 7LD, United Kingdom Received 2 October 2006; accepted 29 January 2007
Abstract Background: The intramedullary ankle nail for tibio-talo-calcaneal (TCC) fusion has been noted in clinical practice to have locking screw holes that do not correspond to the ideal screw placement for bony purchase. Methods: Retrospective review identified 23 patients who had undergone 25 TCC arthrodeses. A retrospective study of the patient radiographs and records were undertaken. Results: The distal PA locking screw hole is significantly too high up the nail itself to facilitate placement of the screw in the optimal position within the calcaneum. The inferior hole should be placed lower and the superior hole placed higher to allow placement of the screws within the calcaneus and talus, respectively. Conclusions: Future designs of intramedullary nailing systems may consider our findings when deciding on screw positions to facilitate the attainment of a solid, pain-free arthrodesis. Future study is required into different designs with different screw options for different operative indications. # 2007 European Foot and Ankle Society. Published by Elsevier Ltd. All rights reserved. Keywords: Ankle; Arthrodesis; Fusion; Intramedullary device
1. Introduction The indications for tibiotalocalcaneal fusion include osteo or inflammatory arthritides affecting ankle and subtalar joint, large deformities of the hind-foot complex and salvage following failed total ankle replacement. It can be associated with high patient satisfaction but the procedure is demanding and complication rates can be significant [1]. It is often preferable to amputation however [2]. The option of intramedullary fixation for tibiotalocalcaneal (TCC) fusion was first described in 1948 by Adams and the exact form has taken many guises since [3]. A longer rod configuration than those tested here is advocated for use in the presence of systemic or localised osteopenia [4]. The ankle arthrodesis nail is a tibiotalocalcaneal reamed fusion device designed for the relief of pain and deformity by means of fusion [5]. It is a second generation nail that * Corresponding author. Tel.: +44 1865 741155; fax: +44 1865 742348. E-mail address: [email protected] (P. Cooke).
affords rigid, load sharing fixation and is available in 15 and 18 cm lengths and varied diameters. The process of tibiotalocalcaneal fusion involves ankle and subtalar arthrotomies through a lateral fibular excision, preparation of the joint surfaces, and then placement of the nail through a plantar incision. Screws are placed proximally into the tibia and, after compression utilising a distal compression device [6], the nail can be locked distally with two lateral screws into the calcaneus and talus with the option of an additional posterior–anterior calcaneal locking screw. The goal is the production of a solid, pain-free arthrodesis in a biomechanically stable and functional position [6]. This study is concerned with the ankle arthrodesis nail and the placement of the screw holes for the lateral–medial (LM) and posterior–anterior (PA) distal locking screws. These had been observed by the senior authors to be suboptimally placed in some cases, especially in patients where loss of normal anatomy from destructive processes such as rheumatoid arthritis or failing arthroplasty had occurred. The hypothesis is that the distal PA locking and distal
1268-7731/$ – see front matter # 2007 European Foot and Ankle Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.fas.2007.01.005
A.F. Young et al. / Foot and Ankle Surgery 13 (2007) 122–125
123
(inferior) LM screw holes are too high (superior), while the proximal (superior) LM screw hole is too low (inferior). A retrospective study of position of screw placement was performed in patients who had undergone TTC fusion.
2. Methods Thirty patients who had undergone tibiotalocalcaneal arthrodesis were identified from computerised theatre records. These procedures were all carried out between June 2003 and March 2004. These patients were operated on by two consultant foot and ankle orthopaedic surgeons. The patient’s records and radiographs were retrospectively reviewed. Twenty-three patients were identified who underwent 25 tibiotalocalcaneal fusions with the Biomet arthrodesis nail. Seven patients were excluded as they had undergone additional procedures. The radiographs were reviewed and the post procedure films were measured to identify the heights of the ankle and subtalar joints (see Fig. 1). This was performed on the lateral films. The patient records were used to identify the length of nail (150 or 180 mm) implant used. The measurements were then calibrated to disregard the magnification (in the region of 15%) inherent to standard film radiographs as the true length of nail was known. The measurements could thus be standardised to give a true reading of the heights of the ankle and subtalar joints at the centre of the bony ankle; the site of the nail implant. This allowed the radiographs to be standardised and thus comparable for the purposes of the study. The heights of the top and bottom of the ankle and
Fig. 1. The points measured on the radiographs. The vertical line simulates the line of insertion of the arthrodesis nail.
Fig. 2. The ideal screw hole placements. The circles represent the ideal screw holes in an arthrodesis nail.
subtalar joints were measured (represented by points A and B, and C and D, respectively on the figure) to correctly identify the centre of the joints. The protrusion of the implant inferiorly from the calcaneus was also measured and recorded (point E). Through this method the calcaneus, talus and ankle and sub-talar joints could be mapped out mathematically. The AP radiographs were ignored for the purposes of this study as they added no further information (Figs. 2 and 3). We assumed the ideal place for placement of a single screw would be the centre of the bone, and the ideal placement for two screws would be one third and two thirds up the height of the bone. This allows for the greatest margin of error to account for normal variation in the human
Fig. 3. Graphical representation of the ideal screw hole placements. The mean is taken to be the ideal placement. Highest to lowest represents the range of the vertical height.
124
A.F. Young et al. / Foot and Ankle Surgery 13 (2007) 122–125
population. The measurements could then be used to give these ideal placements for each patient. These were then compared to the screw hole placement along the ankle arthrodesis nail (15.5, 25.5, and 40 mm from the distal end). The patient sample was divided into two groups to aid analysis; those whose operative indication was primary osteoarthritis and those whose indication was any other. This was designed to exclude any possible bias introduced by the loss of joint and bone height associated with previous operative intervention. In particular, salvage of failed total ankle replacement is hampered by extensive bone loss, osteopenia, and hind-foot pathology [7]. A related t-test was used to determine if any observed differences were significant ( p < 0.05). The surgical indication for the procedure was identified and recorded.
3. Results The study patients underwent tibiotalocalcaneal fusion for arthritis (16 patients), failure of total ankle replacement (5), failed screw ankle fusion (2), or after fracture with severe arthritis (2). The nails were 150 mm (11) or 180 mm (14) in length. The surgical procedures were carried out by two consultant orthopaedic surgeons. The distal hole for the posterior to anterior (PA) distal screw is significantly too high up the nail to facilitate placement within the calcaneus (mean 11.36 mm, range 7.3– 16.1, p < 0.001). The lateral–medial (LM) calcaneal and talar locking screw holes should be placed lower (mean 22.73, range 14.5–32.3, p < 0.01) and higher (mean 43.74, range 23.0–57.9, p < 0.05), respectively to facilitate placement of one within the talus and one within the calcaneus. When the results are divided into the ‘arthritis’ and ‘other’ groups by operative indication then the significance of the differences changes. This may suggest that the screw holes may need to be different according to operative indication. This is displayed in Table 2. The findings suggest that the intramedullary device for arthritis would have screw holes further from the end than a similar device for different operative indications. The table also reveals that only in the case of the LM calcaneal screw in the arthritis group and the LM talar screw in the other group are the present screw holes not significantly different.
4. Discussion Tibio-talo-calcaneal fusion is a technically demanding procedure with high rates of complications. These include high non-union rates [8,9] and wound infection [10]. The nail has evolved over time from a configuration with transverse locking screws only to the current second generation design with a PA screw replacing one of the
Table 1 Mean joint heights (mm)
Tibio-talar joint Sub-talar joint
All subjects
Arthritis group
Other group
53.4 34.1
54.3 35.1
51.8 32.2
Height in millimetres of joints from bottom of calcaneus at point of entry or arthrodesis nail.
transverse screws in the traditional intramedullary nail configuration. This was due to problems of placing a locking screw consistently from the lateral calcaneal body through the rod and into the sustenaculum tali [8]. The distal hole for the posterior to anterior (PA) distal screw is significantly too high up the nail to facilitate placement within the calcaneus (mean 11.36 mm, range 7.3– 16.1, p < 0.001). The lateral–medial (LM) calcaneal and talar locking screw holes should be placed lower (mean 22.73, range 14.5–32.3, p < 0.01) and higher (mean 43.74, range 23.0–57.9, p < 0.05), respectively to facilitate placement of one within the talus and one within the calcaneus. The mean joint heights shown in Table 1 are slightly different to those found in a study Hepple et al. [11] but those measurements had been taken from a normal population and measured in a standardised pattern, whereas the study here is concerned with the heights of the joints in line with the line of introduction of the arthrodesis nail. The measurements in the study undertaken by Hepple et al. would support this study (Table 2). The findings when the results were divided by operative indication would suggest that an intramedullary device designed specifically for patients with severe ankle osteoarthritis would have screw holes at slightly different positions to those for different operative indications. These findings were not significantly different in this study and thus would not warrant separate designs. This should be subjected to further testing. The PA locking screw passes through the calcaneus in a longitudinal manner, providing good bone purchase, and can engage stronger bone such as the sustenaculum tali. In this study, this corresponds to the hypothesised ideal PA screw placement. The placement of a posterior–anterior locking screw has been shown to be significantly stiffer than a lateral–medial screw 8 and thus, we can assume, of great importance. The PA approach affords better purchase in bone, when compared to the LM construct, and helps to neutralize sagittal forces at the fusion site, and thus may help Table 2 Ideal screw hole placements
LM talar screw LM calcaneal screw PA calcaneal screw a
All patients
Arthritis group
Other group
Actual hole
43.7a 22.7a 11.4a
44.7a 23.4 11.7a
42.0 21.5a 10.7a
40 mm 25.5 mm 15.5 mm
Significantly different to actual screw hole height ( p < 0.05).
A.F. Young et al. / Foot and Ankle Surgery 13 (2007) 122–125
to improve fusion rates [3]. The current study has shown that this PA screw hole is sub-optimally placed and should thus be altered in the design of the future ankle arthrodesis nails. The LM screw holes have been shown by this study to be in positions that do not allow the transverse screws to be placed within the centre of the talus or equidistant between the PA screw and the top of the calcaneusm. The position of screw holes in future TCC nail designs should allow for the findings of this study to ensure optimum positioning.
[4]
[5] [6] [7] [8]
References [9] [1] Hammett R, Hepple S, Forster B, Harries W, Winson I. Tibio-talocalcaneal (hind-foot) arthrodesis by intramedullary nailing. Results of 54 cases. Abstract. J Bone Joint Surg 2003;85:246. [2] Miller S. Techniques and prevention of complications for intramedullary nailing arthrodesis of the ankle. Tech Ortho 2001;16:372–80. [3] Mader K, Pennig D, Gausepohl T, Patsalis T. Calcaneotalotibial arthrodesis with a retrograde posterior-to-anterior locked nail as a
[10] [11]
125
salvage procedure for severe ankle pathology. J Bone Joint Surg 2003;85-A(4):123–8. Noonan T, Pinzur M, Paxinos O, Havey R, Patwardhin A. Tibiotalocalcaneal arthrodesis with a retrograde intramedullary nail: a biomechanical analysis of the effect of nail length. Foot Ankle Int 2005;26:304–8. Biomet Inc. Surgical Technique Guide Ankle Arthrodesis Nail. Biomet Inc.; 2005. Quill G. Tibiotalocalcaneal arthrodesis with medullary rod fixation. Tech Foot Ankle Surg 2003;2:135–43. Bruggeman N, Kitaoka H. Arthrodesis after failed total ankle arthroplasty. Tech Foot Ankle Surg 2002;1:60–8. Mann MR, Parks BG, Pak SS, Miller SD. Tibiotalocalcaneal arthrodesis: a biomechanical analysis of the rotational stability of the Biomet ankle arthrodesis nail. Foot Ankle Int 2001;22:731–3. Frey C, Halikus NM, Vu-Rose T, Ebramzadeh E. A review of ankle arthrodesis: predisposing factors to non-union. Foot Ankle Int 1994;15:581–4. Chou L, Mann R, Yaszay B, Graves S, McPeake W, Dreeban S, et al. Tibiotalocalcaneal arthrodesis. Foot Ankle Int 2000;21:804–8. Hepple S, Doling N, Winson I, Glew D. Hind-foot alignment: normal values and their relevance to the tibial-calcaneal arthrodesis. Abstract. J Bone Joint Surg 1999;81:294.