Measurement of surface contact area of the ankle joint

Clinical Biomechanics13(1998)365-370

ELSEVIER

Measurement of surface contact area of the ankle joint Hideji Kura, Harold B. Kitaoka,” Zong-Ping Luo, Kai-Nan An Depurtmwt of Orthopedics,Mayo Clinic und Mayo Foundution, RochrsteKMN 55905, USA

Received3 September1997;accepted23 January1998

Abstract Objective. To delermine the distribution of contact area of the ankle joint with axial loading and in positions of maximal dorsiflexion, plantar flexion, supination, and pronation. We also tested the effects of extrinsic tendon loading and arch instability. Design. Nine cadaveric feet were studied in the intact condition and following transection of ligaments to create arch instability. Background. Assessment of ankle contact in various joint positions and degrees of instability is difficult to accomplish with conventional methods. Methods. Displacement of the talus relative to the tibia was measured with a magnetic tracking device and tibiotalar joint contact from proximity calculations of digitized joint surfaces. Results. Contact area did not change significantly from unloaded condition to 667 N load condition in the medial, central, and lateral zones. Central zone contact area decreased in plantar flexion by an average of 324 mm2 (SD, 16.5mmz) (P = 0.0004). Medial zone contact area decreased in plantar flexion by a mean of 5.5mm’ (SD, 28 mmz) (P = 0.0004), decreased in pronation by an average of 12 mm* (SD, 36 mm’) (P= 0.0086), and increased in supination by an average of 20 mm’ (SD, 26 mm’) (P = 0.0430). Lateral zone contact decreased in plantar flexion by a mean of 124 mm* (SD, 57 mm’) (P = 0.0002). Conclusions. In plantar flexion, there was a decrease in contact area. Loading extrinsic tendons to the foot did not significantly increase ankle contact area, but arch instability caused a decrease in central and lateral zone contact area.

Relevance This technique was used to assessjoint contact characteristics in various loading conditions and will be useful for evaluating the extent to which treatment for ankle or hindfoot problems such as bracing or reconstruction operations restores normal joint contact. 0 1998 Elsevier Science Ltd. All rights reserved. Keywordx Ankle; Biomechanics;Foot; Joint; Joint contact;Rangeof motion

1. Introduction Determination of the magnitude and distribution of contact area as a function of loading level, ankle position, and condition of foot stability is important in understanding the pathogenesis of disorders such as degenerative arthrosis and other abnormalities. Joint contact characteristics in the intact ankle must be defined to provide a basis for comparison with the unstable, injured, or surgically altered ankle. Contact characteristics have been studied extensively in other areas such as the knee and hip, but there is *Correspondenceand reprint requeststo: H. B. Kitaoka, M.D.. Mayo Clinic. 200 First Street SW, Rochester.MN 55905. USA. E-mail: [email protected]

little information available for the ankle joint. Most studies use carbon-black techniques, pressure-sensitive film, or pressure transducers mounted at or below the joint surface [l-lo].

There are some inherent difficul-

ties in these standard testing methods, such as the artifact caused by the use of film between curved joint surfaces, resulting in “crinkle artifact”. Periarticular soft tissue dissection also is required for placement of film and other transducers, and this dissection may alter the stability of the soft tissue envelope. Pressure

transducers provide information for only a limited region of the joint. The purpose of this study was to examine ankle joint contact area and contact frequency in different ankle positions in normal feet and to define the effect of axial loading, extrinsic tendon loading, and arch

026%0033/98/$19.00 $ 0.000 1998ElsevierScienceLtd. All rights resenzcd. PII: SO2hX-0033(08)0001 1-3

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H. Kura et aLlClinical Biomechanics 13 (1998) 365-370

instability. The purpose was also to demonstrate a new method of determining ankle joint features with fewer limitations than other techniques.

2. Methods 2.1. Specimenpreparation Nine fresh-frozen cadaver feet were studied from seven male and two female subjects. The mean age was 70yr (range, 48 to 89yr). Specimens were amputated 20 to 25 cm proximal to the ankle joint and soft tissues were removed from the proximal portion of each specimen. The tibia and fibula were embedded in polymethyl methacrylate by using an alignment apparatus to ensure consistent vertical orientation of the tibia. They were then mounted in an acrylic plastic loading frame in an inverted position for application of axial loads (Fig. 1).

For two rigid bodies in contact with each other, the contact areas can be calculated from the kinematic data and surface geometries. This method has been used to determine the joint contact areas in the patellofemoral joint and glenohumeral joint [15-181. After kinematic measurements of the intact joint, arthrotomy is performed and the undeformed articular surfaces are digitized. The surface was digitized by using one of the magnetic sensors with a sensor tip mounted. The position of the sensor tip relative to the source or another sensor was predefined. By gently touching the tip to the surface without penetration, the position of the point can be determined relative to the sensor or source hxed on the bony segment. By completing this procedure over the entire articular surfaces every 2 mm, the surface was represented by the points. The surface was then batched mathematically in a piecewise continuous fashion. Each digitized surface was repre-

2.2. Testingsequence Two plastic ball-bearing plates were inserted between the foot and loading platform to allow free displacement of the foot in the horizontal plane, thus reducing the effect of shear force as displacement occurred. Three-dimensional position of the talus relative to the tibia and fibula was monitored with a magnetic tracking system (3 Space Tracker System, which is a modification of the 3 Space Isotrack System, Polhemus Navigational Sciences Division, Colchester, VT, USA) [l l-141. This system consists of a system electronics unit, a source, and four sensors. The source and sensors are connected to the system electronics unit. The source emits and the sensors detect low-frequency magnetic fields. The system electronics unit contains all of the analog circuitry to generate and sense the magnetic fields as well as the hardware and software to control the analog circuitry, digitize the signals, and perform the calculations to compute the position and orientation of each sensor relative to the source or another sensor. Large metallic objects between the source and the sensor have the potential of distorting the magnetic field and consequently affect the signals received by the sensor. Therefore, the use of metal parts was minimized or avoided. From the manufacturer’s specification, the system has translational resolution of 0.001 mm/mm-’ and angular resolution of 0.1”. From our previous calibration, the position accuracy depends on the distance from the source to the sensor and is 0.20 mm root-mean-square in translation, and 0.5” root-mean-square in rotation when the sensors are placed 100 to 200 mm from the source.

/ q

Fig. 1. Testing apparatus for application of axial loads and tendon loads. Nylon cables are sutured to tendons, and a static load is applied to each tendon. Note plastic bearing plates between foot and loading platform. (By permission of Mayo Foundation.)

H. Kura et al./Clinical

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Biomechanics 13 (1998) X15-370

sented in the local coordinate system of the bone to which a sensor or source was attached. These surfaces have been represented mathematically using piecewise continuous patches from digitized surface and nodal points, with one surface designated as the shooting surface and the other as the target surface. At any instant, the position of the shooting surface relative to the target surfas: can be determined by using the kinematic data. The proximity, defined as the distance from each nodal point on the shooting surface to the target surface [16] with the positive direction pointing outward on the sh,ooting surface, is then calculated. To more closely present the shooting surface for calculating the joint contact area, more points can be interpolated inside the patches. Each point on the shooting surface is assigned an area based on the area of the adjoining surface subpatches, except for the cases of line contact and point contact. For the ideal case when no error exists, the sum of the areas of nodal points with non-positive proximities will be the contact areas. Owing to inherent measurement errors and surface discretization, using zero as the proximity threshold for defining the correct areas may not be appropriate. A srnall tolerance for the threshold may be needed. In this study, ;an acrylic plastic loading frame was used with a source rigidly fixed to the table surface, the tibia and fibula were mounted to the loading frame, and a sensor was mounted to the talus. The initial position of the foot against the ball-bearing plates was considered the neutral position. A load of 667 N was applied to the footplate to determine the effect of axial loading. This loading level approximates single limb stance. To study the relationship between ankle position and contact area, testing was repeated in positions of maximum dorsiflexion, plantar flexion, supination, and pronation. Loads were applied to tendons that were active during the midstance phase of gait in ratios based on electromyographic and anatomic studies: Achilles (223 N), posterior tibia1 (32 N), flexor digitorum longus (6 N), peroneus longus (17 N), and peroneus brevis (6 N) tendons [19,20]. We then sectioned peritalar soft tissue stabilizing structures, such as the spring ligament, long and short plantar ligaments, tibionavicular portion of the deltoid ligament, tibiocalcaneal interosseous ligament, and medial talocalcaneal ligament, to study the effects of arch instability on ankle contact. Under the 667 N load in the intact foot. the threshold was adjusted to within +0.5 mm until the contact area was 440 mm’ based on previously published data [3]. This threshold was used for all testing conditions for that particular specimen. To illustrate contact locations and systematic changes on the ankle joint surfaces due to loading variations in all nine specimens, the joint was divided into three anatomic zones (Fig. 2). and the contact

Anterior

Medial

Lateral

Posterior Fig. 2. Medial, central, and lateral zones of the ankle joint. Tibia], fibular, and talar articular surfaces were subdivided into 15 regions for determination of contact frequency. Medial, regions 1-3; central, regions 4-12: and lateral. regions 13-15.

area for each zone was assessed.Each zone was further divided into regions (Fig. 2). The contact frequency was defined as the occurrence of contact within any one of the depicted regions, as averaged over all specimens [16]. Discrete data were analyzed with a paired two-tailed t test at a significance level of 0.05. Data are expressed as mean f standard deviation (SD). 3. Results

Total available articular area for the central zone was 922 mm2 (SD, 120 mm2), medial zone was 178 mm2 (SD, 66 mm’), and lateral zone was 308 mm* (SD, 60 mm’). The contact area of the central zone decreased from the unloaded condition (mean, 442; SD, 146 mm’) to the loaded condition (mean, 439; SD, 10 mm’) (not significant). The contact area of the medial zone increased from the unloaded condition (mean, 81; SD, 27 mm’) to the loaded condition (mean, 99; SD, 40 mm’) (not significant). The contact area of the lateral zone decreased from the unloaded condition (mean, 178; SD, 78 mm’) to the loaded condition (mean, 171; SD, 79 mm’) (not significant). The central zone contact area decreased in dorsiflexion an average of 39 mm* (SD, 229 mm’) (not significant), decreased in plantar flexion an average of 324 mm7 (SD, 165 mm’) (P= 0.0004) decreased in pronation an average of 96 mm’ (SD, 163 mm’) (not significant), and decreased in supination an average of 150 mm’ (SD, 221 mm’) (not significant). Medial zone contact area increased in dorsitfexion an average of 7 mm’ (SD, 42 mm’) (not significant), decreased in plantar flexion an average of 55 mm’ (SD, 28 mm’) (P = 0.0004), decreased in pronation an average of 42 mm’ (SD, 36 mm’) (P = 0.0086) and increased in supination an average of 20 mm* (SD, 26 mm’) (P = 0.0430). Lateral zone contact area decreased in dorsiflexion an average of 21 mm’ (SD, 70 mm’) (not

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H. Kura et aLlClinical Biomechanics 13 (1998) 365-370

condition of foot stability is important in understanding the pathogenesis of degenerative arthrosis and other abnormalities. Changes in foot stability or joint alignment or both may lead to degenerative changes and ankle pain [21, 221. This is the first study to demonstrate contact features of the ankle joint that does not have supporting structures such as ligaments and joint capsules dissected, which is a limitation of techniques using thin pressure transducers or pressure-sensitive paper. In a report of a study by Calhoun et al. [l], in which they used pressure-sensitive film in five cadaveric feet, the authors noted a decrease in contact area with inversion, eversion, and plantar flexion positions and an increase in dorsiflexion. They noted that inversion increased the medial facet contact area and eversion increased the lateral facet contact area. There was also an increase in contact area with loading. The present study did not demonstrate a significant increase in contact with loading, but there was an increase in medial zone contact in supination (inversion). The present study also demonstrated a decrease in contact area with plantar flexion in all three zones. Beaudoin et al. [2] used pressure-sensitive film to study the ankle, talonavicular, and posterior facet and subtalar joint in five specimens. There was no signifi-

significant), decreased in plantar flexion an average of 124 mm* (SD, 57 mm’) (Z’= 0.0002), increased in pronation an average of 5 mm* (SD, 39 mm’) (not significant), and decreased in supination an average of 44 mm2 (SD, 63 mm’) (not significant). There were fewer zones of contact in certain joint positions such as plantar flexion (Table 1). Contact frequency shifted laterally in pronation and medially in supination. Contact frequency did not change appreciably with loading of tendons (Table 2). There was an increase in central zone contact of 96 mm2 (SD, 156 mm”) (not significant), increase in lateral zone contact of 81 mm2 (SD, 95 mm2) (not significant), and increase in medial zone contact of 8 mm2 (SD, 41 mm”) (not significant). Arch instability decreased contact in all zones (Fig. 3(B)) compared with normal (Fig. 3(A)). The central zone contact area decreased an average of 211 mm2 (SD, 252 mm”) (P = 0.0364) lateral zone contact decreased 103 mm* (SD, 83 mm’) (P = 0.0053), and medial zone contact decreased 32 mm2 (SD, 69 mm”) (not significant). 4. Discussion Analysis of the magnitude and distribution of contact area as a function of loading level, ankle position, and Table 1 Ankle contact frequency, % (N = 9) Condition

Neutral, 0 N Neutral, 667 N Pronation Supination Dorsiflexion Plantar flexion

Regions in control (no.1

12 68 72 44 61 19

Contact regions Medial

Lateral

Central

1

2

3

4

5

6

7

8

9

10

11

12

13

14

1.5

4 3 3 7 8 5

3 0 4 4 5 0

3 3 3 2 3 0

8 10 I 14 11 10

7 9 3 11 5 5

1 1 1 2 1 5

7 9 10 4 7 10

10 9 3 4 8 10

6 3 1 2 3 5

8 12 I 2 8 10

10 10 8 9 10 5

7 3 5 4 3 0

10 10 11 14 10 15

8 10 11 11 10 10

8 9 10 7 7 5

Table 2 Ankle contact frequency, % (N = 9) Condition

Neutral, 111 N no tendon load Neutral, 111 N tendon load Flatfoot, neutral 111N tendon load

Regions in control (no.)

Contact regions Medial

Lateral

Central 6

7

8

9

10

11

12

13

14

9

3

8

I

5

8

9

7

9

9

8

7

10

6

6

6

7

7

7

6

8

8

7

14

11

2

4

4

2

2

9

4

14

11

7

1

2

3

14

4

1

3

8

84

6

5

4

44

7

4

2

4

5

15

H. Km-a et ul.lClinical

Biomechanics 13 (1998) 365-370

cant change in ankle contact area in dorsiflexion and plantar flexion, unlike the present study. The ankle joint contact was similar before and after subtalar fusion. Ramsey and Hamilton [3] studied changes in tibiotalar contact area with lateral displacement of the talus by using carbon-black transference techniques. Ting ef al. [4] found a decrease in ankle contact after pinning the talocalcaneal ,joint. Michelson et al. [5] studied the effects of fibular osteotomy on ankle contact and found that there was no change in the contact area. However, on sectioning the deltoid ligament there was a decrease in contact area. They used a carbon-black suspension photographic technique. Vrahas et al. [6] used pressure-sensitive film to determine peak contact stress in ankles with simulated ankle malunion. Macko et al. [7] noted a decrease in contact area of the ankle with increasing size of a posterior malleolar fracture fragment. Libotte et al. [8] and later Paar et al. [9] noted a decrease in contact area with plantar flexion and dorsiflexion. Kimizuka et al. [lo] used a silicone rubber casting technique for determining contact area and also pressure-sensitive film of the superior surface of the trochlea of the talus and found increasing contact area with loading. The areas of contact were primarily anterior and lateral, not

Anterior

(a)

369

medial and posterior. They noted that with increasing tibiofibular diastasis there was a decrease in contact area and increase in peak pressure in the four feet tested. The current study demonstrated a method that is useful for defining contact features of the ankle joint in various ankle positions in both stable and unstable conditions. It will be applicable for studying the effects of simulated joint abnormalities such as the effect of fracture malunion. It will also be useful to define the mechanical efficacy of reconstruction operations in restoring joint contact. Furthermore, it will be applicable in studying to what extent operations such as hindfoot arthrodesis and arthroplasty affect the ankle. The contact area did not appreciably change with increased loading, but decreased with plantar flexion and flatfoot conditions. The contact frequency shifted laterally with pronation and medially with supination. These findings are, for the most part, consistent with previous studies with pressure-sensitive film. There are limitations to this study. The feet were initially loaded axially to simulate the standing-at-ease posture. Further investigations could be performed to simulate other phases of gait. Owing to measurement error it is difficult to calculate contact area accurately, and in this study the contact area was estimated by adjusting the proximity threshold to match the contact area measured directly. Contact pressure could not be directly measured. The present study did not validate the accuracy of the technique, but this could be the subject of further investigations. Acknowledgements

Medial

D

Contact

We acknowledge the support of the Arthritis Foundation and National Institutes of Health, with the assistanceof T. K. Ahn, M.D.

Posterior References

Anterior

Medial

m

Contact

Posterior

Fig. 3. A typical contact pattern of the ankle joint surface is shown in the intact foot (A) and in the flatfoot conditions (B) under 667 N load. Darker region corresponds to articular contact and lighter region indicates no contact between tibia and talus.

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