THE EFFECT OF CALCANEAL OSTEOTOMY ON CONTACT CHARACTERISTICS OF THE TIBIOTALAR JOINT
November 21st, 1994
Adrian Fairbank, MbBCh, Mark S. Myerson, MD,
Paul Fortin, MD, and Janet Yu-Yahiro, PhD
Summary
To investigate the effect of calcaneal osteotomy on tibiotalar joint contact patterns in a normal and a flatfoot model, we axially loaded 11 fresh frozen cadaver legs with 700 N and quantified tibiotalar joint contact characteristics using Fuji superlow pressure film. For each testing sequence, the unconstrained foot was loaded in neutral, 10o dorsiflexion, and 10o plantarflexion. Tibiotalar joint loading was performed both with a materials tester (Group 1) and manually using a customized jig (Group 2). After calcaneal osteotomy, the calcaneus was translated medially or laterally by 10 mm, and the testing sequence was repeated. Each of these testing sequences was repeated on a flatfoot model simulated by soft-tissue sectioning.
Images were analyzed using a Bioquant Video system for Group 1 and an Orthographics Digitizing Tablet for Group 2. For Group 1, there was no difference in the quantitative tibiotalar joint contact characteristics with the exception of all parameters evaluated with the foot in plantarflexion (p = 0.062). For Group 2, in the normal foot, a lateral translational osteotomy had no effect, but a medial translational osteotomy was associated with a change in the tibiotalar contact (p = 0.05). The flatfoot model (Group 2) was associated with a qualitative shift of pressure laterally and a quantitative alteration of the contact area in all positions when compared to the normal ankle (p = 0.06). A medial translational osteotomy altered this contact area (p = 0.001).
We concluded that hindfoot alignment can adversely affect tibiotalar joint contact characteristics. A calcaneal osteotomy may be a useful alternative to tibiotalar arthrodesis in cases of early tibiotalar arthritis secondary to severe varus or valgus hindfoot deformity.
Introduction
During ambulation, both the ankle and subtalar joints can compensate slightly for small degrees of angular deformity of the tibia or hindfoot,1 and there is a correlation between angular deformity of the tibia and the contact characteristics of the tibiotalar joint.2 Similar changes in contact occur associated with deformity of the hindfoot.3 In vitro talocalcaneal fusion with excessive plantar flexion, varus, or valgus has been shown to adversely affect the contact characteristics of the ankle joint.4
The subtalar joint plays a significant role in maintaining the normal relationship between the tibia and the talus. Inman5 suggested that the subtalar joint, which acts a torque converter, can compensate for both varus and valgus deformities of the tibia. The orientation of the calcaneus with respect to the longitudinal axis of the tibia is an important determinant of subtalar and transverse tarsal joint motion. Eccentric alignment of the calcaneus lateral to the axis of the tibia and ankle joint creates a valgus thrust on the subtalar joint at heel strike which, in turn, leads to pronation through the transverse tarsal joints.6 Although the inherent shape of the tibiotalar joint is believed to favorably affect its contact characteristics and protect against degenerative changes,7,8 this is not supported by clinical experience. Long-standing hindfoot valgus9 as well as cavovarus deformity10 can lead to degenerative changes in the ankle joint. Significant varus or valgus deformity of the hindfoot may therefore adversely affect the contact characteristics of the tibiotalar joint.
Although calcaneal osteotomy has been recommended as a treatment for both hindfoot varus11,12 and hindfoot valgus,13-15 it is not well understood to what extent alteration of the position of the hindfoot may positively or negatively affect the loading characteristics of the tibiotalar joint. The purposes of this study, therefore, were to identify the in vitro contact characteristics of the tibiotalar joint and to determine whether alterations in the position of the calcaneal tuberosity, with respect to the tibial axis, affect ankle joint contact characteristics. We proposed that medial or lateral calcaneal osteotomy causes a shift of the mechanical axis of the tibiotalar joint, resulting in a change in the contact area of this joint.
Materials and Methods
Eleven fresh cadaveric below-knee specimens were harvested and deep frozen to -20o and stored in polyethylene bags until required for use. They were thoroughly thawed at room temperature on the day of study, and a transverse osteotomy was performed at the level of the proximal tibial metaphysis. Specimens were inspected to ensure that at least 10o of ankle dorsiflexion and no hindfoot deformity was present, and radiographs were obtained to exclude any abnormality.
A platform was constructed to support the limb, provide access to the ankle joint, and allow minimally constrained sagittal plane motion. Since patterns of contact in the tibiotalar joint have been shown to be dependent upon ankle position,3,4 freedom of motion in the sagittal plane was provided. The ankle was therefore loaded in neutral, dorsiflexion, or plantarflexion as previously recommended.7,8 A custom-made intramedullary steel rod 10 mm in diameter was inserted (press fit) into the tibial canal and attached by a rigid coupling fixture to the loading devices.
Pressure-sensitive transducers were made of Superlow Fuji Pressure-Sensitive film (Fuji Photo Film, Ltd., C. Itoh and Co., New York, NY) and were hand-cut to accurately cover the dome of the talus. Each transducer was modelled according to the anterior transverse dimensions of the trochlea of the talus. Since these transducers are moisture-sensitive, each was placed into a condom to prevent contact with moisture from the ankle joint and surrounding thawed tissues.
The soft tissues were removed from the anterior aspect of the ankle to expose the joint and insert the pressure-sensitive film. A smaller posterior ankle arthrotomy was performed to withdraw the film reproducibly into the joint and to ensure that the film was consistently positioned. The transducers in the bags were noted to be held firmly positioned during loading. All the ankle joint ligaments were preserved during the anterior ankle dissection. The ankle joint was manually distracted and the transducer was carefully inserted and centered on the talar dome.
Two different methods of loading of the tibiotalar joint were evaluated. In Group 1 (five specimens), an MTS 858 Bionix Materials Testing Machine (Minneapolis, MN) was used to apply a static axial load of 700 N. This load was applied statically with the ankle positioned in neutral, 10o of plantarflexion, and 10o of dorsiflexion (Fig. 1). In Group 2 (six specimens), dynamic manual loading was performed with a customized platform using a constant weight of 100 pounds (45.5 kg). The platform was designed with a slot to allow minimally constrained sagittal plane motion only. A similar construct has been used previously to evaluate the loading characteristics of the tibiotalar joint.16 The device was manually loaded, and the tibia was manually moved 10 times through a cycle of motion defined by the excursion of the intramedullary rod, by holding the protruding portion of the intramedullary rod above the coupling fixture (Fig. 2). The loading in Group 2 was also performed with the ankle positioned in neutral, 10o of plantarflexion, and 10o of dorsiflexion.
After this loading sequence (the normal foot), the skin and soft tissue were removed from the lateral aspect of the hindfoot to expose the calcaneus. Care was taken to identify and protect the ligamentous support of the subtalar joint. An oblique osteotomy was made through the tuberosity of the calcaneus at 45o to its longitudinal axis and perpendicular to its lateral wall. The soft tissues on the medial aspect of the calcaneus were loosened to allow 10 mm of medial or lateral translation. The position of the tuberosity was then maintained by securing it with two threaded Steinmann pins. The specimen was loaded in each of the three test positions (neutral, 10o dorsiflexion, and 10o plantarflexion) with the calcaneus in the neutral position and after medial and lateral osteotomies. A simulated flatfoot deformity was then created by transecting the posterior tibial tendon, distal portion of the superficial deltoid, plantar fascia, spring ligament, long plantar ligament, and talonavicular joint capsule.17 The specimen was loaded in each of the three test positions with the tuberosity reduced and translated medially and laterally by 10 mm. To ensure reproducibility, the loading sequence for each set of experimental conditions in each position was repeated three times. These sequences were repeated on all specimens for both Groups 1 and 2.
The transducer prints for both Groups 1 and 2 were digitized using a video imaging system.18-20 However, the method of evaluating these two groups was different. For Group 1, the contact area and color density of the contact area was digitized using BioquantR, a semi-automated quantitative video image analysis system. This system takes an image of the contact pattern on video and evaluates its color, density, shape, size, and area, all of which may be measured quantitatively. Results of Group 2 were analyzed using an Orthographics digitizing tablet. The outline of the borders of the pressure-sensitive contact areas were marked and measured (cm2). To ensure reproducibility of this data, five measurements of the marked area were made, each of which was found to be consistent (Pearson correlation coefficient = 0.975). Qualitative evaluation of the position of the contact area on the film was performed in a blinded fashion by one of us (ACF). Six sets of data were generated for Groups 1 and 2, including measurements with the calcaneus in the neutral position and after medial and lateral osteotomies for both the normal and the flatfoot. The pooled data for each of these six sets were also subdivided into the three test positions for the tibiotalar joint (Fig. 3). A one-way analysis of variance (ANOVA) with a Scheffe was used to determine significant differences for each of these pooled sets of data. Using the ANOVA, area and pressure were each evaluated separately as a dependent variable.
Various parameters, including the area of tibiotalar contact pressure, changes in the density of this contact, and area shifts, were evaluated. Each of these three parameters was analyzed for the normal foot, flatfoot, and foot after medial or lateral osteotomy. The positions of neutral, plantarflexion, and dorsiflexion were evaluated separately in each of the positions described above. The five trials for each specimen were averaged, and the data were pooled for all specimens in a group.
Results
Group 1
Using the Bioquant system, the tibiotalar contact area was mapped out, and the area was measured for each trial in every position tested. The mapping was performed by defining a color threshold on the Fuji Film that the system recorded in units (micrometers). The absolute area was determined in this manner but, in this phase of analysis, no quantitative or qualitative shift in area was examined. There was no significant difference between any of the parameters evaluated with respect to changes in absolute area. Since this analysis failed to account for minor changes in pressure and, hence, color, the percentage of the area covered by the threshold color was found to be more accurate than the absolute area. All pressure phenomena were now examined, since a percentage of an absolute area covered by the threshold color was determined. With this method, a significant difference (p = 0.062) between all the groups tested in plantarflexion was identified. Although there was no other statistically significant difference in any overall group, there was a visible and quantifiable difference in the area between the medial and lateral osteotomy, but only in the flatfoot model.
A change in density of the tibiotalar joint contact for all sets of pooled data was measured by determining the differences in intensity of the color of Fuji film contact. The same methodology used for determining area above was used to evaluate pressure. The area of contact was circled and the density measurements were averaged. Both absolute pressure measurements and percentages of pooled data were obtained for each subgroup. With the exception of a significant difference (p = 0.062) between the medial and lateral osteotomy in the flatfoot in plantarflexion, no other significant differences were identified.
Medial or lateral shifts in the area of tibiotalar contact were evaluated. These shifts were analyzed by defining the center of the area using x-y co-ordinates and shifts in the area measures using the Pythagorean theorem. There were no significant differences in the spatial area of tibiotalar contact for any of the parameters evaluated.
Group 2
There were significant changes in the contact area recorded using the Orthographics digitizing tablet. In the normal (non-flatfoot) model, a lateral translational osteotomy had no significant effect on contact area in any position in which the ankle was tested, but a medial translational osteotomy significantly altered the area of contact (p = 0.05). This was interpreted as a medial shift in the tibiotalar contact. The flatfoot model was associated with a qualitative shift of the contact area laterally. In this flatfoot, a quantitative change in the area of contact occurred in all positions of the ankle tested when compared to the normal ankle (p = 0.06). There was a qualitative change, and a significant quantitative alteration, in the areas of contact in the flatfoot model when comparing the lateral with the medial translational osteotomy in all positions of the ankle tested (p = 0.001).
Discussion
Tarr et al2 demonstrated that angular deformities of the distal third of the tibia produce significant changes in contact patterns in the ankle joint. They and other authors21-24 have demonstrated that alteration in joint alignment is responsible for subsequent degenerative arthritis. Similarly, deformity of the subtalar joint and hindfoot has been shown to have an adverse effect on the tibiotalar joint, and long-standing hindfoot varus or valgus causes predictable alteration in the mechanics of the tibiotalar joint, leading in many instances to degenerative arthrosis. The subtalar motion that occurs during the stance phase of gait is believed to reduce the rotatory stress transmitted to the tibiotalar joint.25 This may explain why degenerative arthrosis of the tibiotalar joint occurs in cases of long-standing hindfoot varus or valgus, when minimal motion in the subtalar and transverse tarsal joints occurs. Although valgus tilting of the tibiotalar joint associated with prolonged planovalgus deformity is also due to rupture of the deltoid ligament,26 the valgus thrust of the subtalar and transverse tarsal joints and the alteration in the mechanical axis of the lower extremity with this prolonged hindfoot valgus may also exacerbate this ankle deformity.
The failure of soft-tissue surgery to predictably restore alignment and to provide lasting improvement for varus and valgus deformity of the hindfoot has led to the conclusion that arthrodesis is required to achieve a satisfactory outcome. Although attempts to correct hindfoot valgus by arthrodesis have met with success, these arthrodeses have significant deleterious effects on adjacent joints, particularly the ankle. In a recent study of triple arthrodesis for older adults, Graves et al27 reported that 14 of 18 patients demonstrated subsequent degenerative changes in the tibiotalar joint and that, in seven of these patients, these degenerative changes were progressive. In a separate study on triple arthrodesis in adults, Bennett et al28 reported a 50% incidence of tibiotalar arthritis after triple arthrodesis, as have other authors.29,30 It is not known, however, to what extent hindfoot deformity will alter the contact characteristics of the tibiotalar joint, ultimately leading to degenerative arthrosis.
In an attempt to avoid arthrodesis, many authors have recommended performing various osteotomies of the calcaneus to improve the hindfoot alignment.13-15,20,31,32 Silver and co-workers13,14 credit the earliest report of calcaneal osteotomy to Gleich in 1898 when he reported on a translational osteotomy of the calcaneus for the treatment of flatfoot. The use of a medial displacement osteotomy of the calcaneus has been previously described to treat the flexible flatfoot. Koutsogiannis15 proposed that this osteotomy altered the biomechanical axis of the lower limb, thereby reducing the valgus thrust on the hindfoot. Other authors11,12,32 have treated the varus hindfoot deformity with a valgus osteotomy of the calcaneus.
This study was designed to evaluate tibiotalar joint contact characteristics and to determine whether a translational calcaneal osteotomy in the normal and flatfoot model would affect this contact. Our hypothesis was that a translational osteotomy of the calcaneal tuberosity would alter the weight-bearing axis of the lower extremity, thereby altering, possibly beneficially, the contact characteristics of the tibiotalar joint. To evaluate this hypothesis, the contact characteristics of the ankle were examined using both static (Group 1) and dynamic loading (Group 2). We identified significant changes in the areas of contact both qualitatively and quantitatively using the dynamic manual method of loading. Significant alteration of tibiotalar contact occurred in the static method of loading, but only between the medial and lateral translational osteotomy with the foot in plantarflexion. Furthermore, in Group 1, these changes were identified only in the flatfoot model.
The discrepancies in the results between Groups 1 and 2 were the consequence of either a flaw in the testing methods or inherent differences in the static and dynamic means of evaluating the joint. Although we used previously described static2,4,25 and dynamic16 methods of loading for the tibiotalar joint, it was unfortunately impossible to use the same video image analysis system for both groups. The varied results obtained may therefore simply be due to differences in the manner in which the data were quantified using the Bioquant video image analysis (Group 1) and the Orthographics digitizing tablet (Group 2). The Superlow Fuji film used is load-rate dependent, and the load was therefore quantitatively applied over a standardized period of 3 seconds in Group 1. Whether these and other variables, such as shear forces, had any bearing on the differing results in Groups 1 and 2 is not clear. It is difficult to state whether or not dynamic loading of the ankle is more representative of weight-bearing forces. A cadaveric biostatic model (Group 1) cannot simulate conditions of bearing weight, particularly with one load application. Although the dynamic model (Group 2) may be more representative of weight-bearing, error could have been introduced in the manual manipulation of the intramedullary rod. Nevertheless, the patterns of contact for each of the trials in Group 2 were highly reproducible.
In conclusion, these results do not fully support our hypothesis. Although osteotomy has been proven to be beneficial in the treatment of hindfoot deformity, further clinical study will be required to determine whether osteotomy does indeed delay the advent of degenerative arthrosis in the tibiotalar joint.
References
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Figure Legends
Fig. 1. A 700 N static load was applied on a Bionix 858 Materials Tester to the ankle positioned in neutral, 10o of dorsiflexion and 10o of plantarflexion.
Fig. 2. A manual dynamic load of 100 lbs (45.5 kg) using this customized jig was applied to the performed with the ankle positioned in neutral, 10o of dorsiflexion, and 10o of plantarflexion.
Fig. 3. Representative examples of the pattern of tibiotalar contact is demonstrated in the flatfoot model with the calcaneus in neutral position (A) and after a lateral (B) and a medial (C) translational osteotomy of the calcaneus. The values expressed under each pressure diagram represents the area in cm2. N, neutral; D, dorsiflexion; P, plantarflexion; med, medial; lat, lateral.
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