The Functional Anatomy of the Ankle Joint Fosters - Coursework Example

ENT536 Engineering Theory work Assignment 2 Dr DM O’Doherty Based on the stress distribution in the ankle joint, discuss whether the ideal requirements of a total ankle replacement system have been met in any of the current designs. Introduction The functional anatomy of the ankle joint fosters in normal condition a plantigrade and bipedal ambulation. In normal condition, the ankle joint acts as a functional unit with the respective subtalar joint. During different phases of gait, the intact ankle joint is anatomically and mechanically suited to efficiently dissipate compressive, rotatory, and shear forces of impact and motion while simultaneously adapts to ground reaction forces resultant from weight bearing. These can be facilitated by anatomic factors such as large articular contact area, which can confer inherent stability under a static load. However, the dynamic stability to the joint can be attributed to the ligamentous support and balanced muscular forces acting around the joint. Although end-stage degenerative joint disease in uncommon in the ankle joint, in contrast to the previously offered ankle arthrodesis, with the newer designs of total ankle arthroplasty implants, the later has become a viable alternative. However, given the complex mechanism of the joint in terms of force distribution, any successful implant must be congruent with the biomechanical properties of this unique joint. To this end, stress distribution and other mechanical forces are the most important considerations, and in this assignment, some current total ankle replacements systems will be investigated as to whether they conform to the ideal requirements of stress distribution (Alvine, 2000). Studies have supported the clinical choice of total ankle replacement despite its complications since in comparison to arthrodesis, the ideal patients undergoing indicated total ankle replacements can experience a near-normal gait, greater range of movement, symmetrical timing but a slower gait, and restored ground reaction pattern. In actual clinical conditions, thus stress distribution across the implant becomes the most important engineering issue to be considered while choosing an implant. This is important more so, given the fact that there is indeed a higher reported incidence of frequent failure of the ankle implants. These have been ascribed to the designers and surgeons inability to reconstruct and restore the stabilising ligaments, to a poor simulation and reproduction of the normal mechanics of the joint, and due to these reasons, leading to a lack of involvement of the subtalar joint while the entire ankle complex need a coupled pattern of motion. This makes the total joint replacement challenging, but also indicates that there is space for improvements in implant design and respective techniques (Falsig et al., 1986). There is a variety of implants available, and this discussion will restrict itself to the new generation implants available and presently in use. Mechanical Aspects of Stress While there are several implants available, most of the total ankle replacements systems bank upon the available bone support. This is particularly significant in patients with increased risks such as nature of the baseline disease, depletion of bone mass of the surrounding bone due to osteonecrosis, disease of long duration, steroid use, and chronic inactivity. The unique nature of the physiologic force vectors surrounding the newly constructed joint makes it mandatory to study the stress distribution pattern within the newly constructed joint. Studies involving systems that remove the cortical shell of talus indicate that stress distributions across the talar and tibial component interfaces hold the key to success of any total ankle arthroplasty systems. Studies through three dimensional models of implanted prosthesis demonstrate that removal of talar cortical shell in order to accommodate the talar component of the total ankle systems produces increased stress on the remaining talar bone. This accentuates the angle of stress distribution pattern in the prosthetic joint, and this consideration should be crucial in success or failure of any total ankle system. It has been observed that, in the prosthetic ankles, the bone strength is reduced markedly away from the articular surface in these joints. Moreover studies have shown that the talar bone is 40% stronger than the distal tibial bone which should at least equal or exceed 20 MPa to ensure success of the replacement system. Moreover the strongest bone is eccentrically placed and hence not evenly distributed across the tibia and is posteromedial in disposition. Thus the force generated by heel strike may produce a pivot point leading to subsidence of the tibial component into the surrounding bone which is anterolateral (Saltzman et al., 2000). The reason these are significant factors worth considering while choosing a total ankle system is that the forces at the lower extremity are actually large. Due to the principles of leverage, these forces magnify the force of the bodyweight, and this magnification is particularly most accentuated at the ankle joint. Mechanically, the forefoot metatarsal pad is located at a greater distance than ankle to hind foot with the reference point being the fulcrum of the ankle joint. This results into a longer anterior arm of the lever. This necessitates a very large tensile force to overcome the body weight during ambulation. This requirement due to the longer lever arm of the forefoot is met by the Achilles tendon tensile forces, large enough to match this mechanical demand (Raikin et al., 2000). It is easy to conceive that this will generate very high compressive forces at the ankle. Any total ankle replacement system must provide enough consideration to these compressive forces. In any such model, as has been shown by Bandak et al. (2001) that the bony components such as tibia, fibula, calcaneus, and talus together with major ligaments, cartilages, and soft tissues in the plantar foot would behave as linearly viscoelastic material. In such models, the predicted stress distribution demonstrates localised stress at anterior talofibular and deltoid ligaments (Bandak et al. 2001). On the other hand, Spears et al. (2007) in their study involving a viscoelastic model of heel CT images in order to quantify the stress distribution involving varying foot inclinations, loading rates, and forces indicated that the highest compressive stress could be predicted just inferior to the calcaneal tuberosity. Contrary to expectation, during heel strike, it appeared that the internal stress was higher than the pressures at the plantar aspect. This research also indicated that with increasing loading rates, there would be a decrease in the contact area and corresponding strains (Spears et al. 2007). The more interesting finding was that increase in tissue stress was commensurate with an increase in the internal stress which was far more than the rise in plantar pressure. These findings can be very significant in assessing the internal stress in total ankle systems since there was an observation of higher general stress levels at inclined loading of the ankle with a posterior shift of the induced stress (Spears et al., 2005). Bony Anatomy Consideration of unique bony anatomy is important in analysing the internal stress within the ankle joint. This bony anatomy confers a high degree of stability and congruence in the loaded joint. The three bones, namely, tibia, fibula, and the talus support three sets of opposing articular surfaces. These along with the ligaments surrounding the ankle ensure restrained movements between talus and the rest of the mortise leading to continuously changing axis of rotation during its movement from maximal plantar flexion to maximal dorsiflexion with slight widening from posterior to the anterior. Therefore, at loading, the joint has a smaller area of contact between opposing articular surfaces, for example, at 500 Newtons of load the contact area averages 250 mm2. This suggests with equal loading the spatial average contact stress will be higher than in other large joints such as knee or hip (Kempson et al., 1975). It is evident that the forces across the ankle joint are not markedly affected by the design of the prosthesis; the critical factor remains the surface area of contact between the components of the ankle system and the bone resected. What actually is critical in the prosthetic design is pressure or stress caused by the prosthesis on the bone. This leads to the proposition that the more the surface area less is the stress (Lunberg et al., 1989). Although the total surface area of the ankle joint is 12 cm2, much of this is expended in the lateral gutters and on the large anteroposterior dome of the talus. This indicates that the prosthetic design is crucial since much of this surface may not be available for prosthetic support. The design of the total ankle system must incorporate the consideration that the available area of talus for the fixation of prosthesis is bone. With resection of the dome of the talus, the area for engagement of the prosthesis is reduced, and the available surface area is almost half of that in comparison to the upper tibia at the knee. On a larger surface area, the compressive force at the knee is three to four times that of the body weight. On the contrary, during movement, the compressive forces at the ankle may be 5.5 times that of the body weight due to the much smaller surface area available leading to increased load per unit area (Michelson et al., 2000). Implants For the above reason, most of the implants have a keel designed to expand the surface area. This also has the added advantage of reduction of force per unit area. However, with the prosthesis with a provision for a keel poses the problem of preservation of sufficient support bone due to the small size of the talus. Moreover, the subtalar joint is in close proximity to the talus, limiting the distal expansion of the keel. The keel is also limited medially and laterally due to the narrowness of the talus. As has been indicated earlier, the stress across the ankle is eccentric and thus is not equally or uniformly placed across the support surfaces of the prosthesis. This off-centre force across the prosthesis leads to an intrusive force on one side and an elevation force that tends to lift off on the side opposite to it. The movement of the talus has a glide component and hence the talar component of the prosthesis would lead to a shear force leading to enhancement of stress on the cancellous bone that underlies the talar prosthesis. A study has shown that keel in the base plate designs may be helpful to resist these eccentric and shear stresses. Of all the designs of total ankle arthroplasty studied, the worst stress management was observed with tibial base plates with no understructures (Saltzman, 2000). Current Designs The design rationale of INBONE total ankle replacement system will be discussed here. There is problem of poor fixation in ankle prosthesis in previous systems. As has been indicate earlier, insertion of a large-stemmed prosthesis is difficult in the ankle joint. INBONE technology allows a patented modular stem which can create a custom length stem. This averts the problem of leverage at the ankle from the tibial component through possibility of a superior fixation without any possibility of weakening the anterior portion of the tibia. The talar system of INBONE allows maximum length without a chance of violation of the subtalar joint (Reiley, 2008). Alternatively, a specially designed calcaneal stem may be used. As evident from the discussion of ankle motion vectors, a vertical fixation would attempt to even distribution of the stress, leading to more stability and less era and tear. For even distribution of stress that usually tends to be eccentric, it is important that all bony components of the mortise remain intact. Also the ligamentous structures that bear most of the stress also are important for this purpose. However, in contrast to other systems, the greater fixation does not need a larger tibial plateau, so there is no need to remove all of the medial malleolus and fibula. Poor coverage of inferior weight bearing surface is known to lead to component subsidence (Gill, 2004). In the INBONE, the talar dome covers the entire talar cut surface. This means anatomically, the resected talus is identical to the INBONE talar dome. This provision allows a large surface area of talar component, leading to reduction of stress per unit area. This also allows matching of natural saddle geometry of the ankle, which again allows a near normal stress distribution and hence stability to applied stress. The polyethylene durability of the components is also determined by stress. This is a determinant of wear and tear that can be minimised with reduction of surface stresses and stress gradients. This design also allows distribution of surface stresses over the largest possible area accomplished by the INBONE. Stress gradients are minimised through reduction of stress differences across short distances. The INBONE thick poly has less stress gradients and minimised stress delaminating wear since the thicker poly can act as a cushion and may spread out the high stress to lower gradients of stress. Mechanically, under large compressive load on the joint, the friction coefficients between opposing surfaces may prevent or at least the relative sliding (Wood, 2003). Conclusion The INBONE total ankle replacement system conforms to ideal requirement of stress distribution in the ankle joint. As has been discussed earlier regarding different factors in stress distribution of the ankle joint, the INBONE matches nearest requirements. Adjustable superior fixation provides adequate support for talar and tibial components. Vertical fixation allows minimal removal of bone and hence provides better stability. Talar components tend to match normal anatomy and ensure normal stress distribution. This helps to create natural and stable ankle anatomy and motion. Use of thick poly in this system increases surface area leading to less wear component subsidence, and minimal wear. Thus is fulfils the most of requirements of an ideal total ankle replacement system. References Alvine, F (2000) Total ankle arthroplasty. In: M Myerson, ed, Foot and Ankle Disorders, vol. 2., Philadelphia, WB Saunders, pp. 1085–1102. Bandak, F.A., Tannous, R.E., Toridis, T. (2001) ‘On the development of an osseo-ligamentous finite element model of the human ankle joint’. Int J Solids Struct, 38:1681–97. Falsig, J., Hvid, I., Jensen, NC., (1986). Finite element stress analysis of some ankle joint prostheses. Clin. Biomech. 1:71 –76. Gill, LH., (2004). Challenges in Total Ankle Arthroplasty. Foot & Ankle International; 25 (4): 195-207 Kempson, GE., Freeman, MAR, Tuke, MA, (1975). Engineering considerations in the design of an ankle joint. Biomed. Eng. 180: 166–171. Lunberg, A., Svensson, OK., Nemeth, G., Selvik, G., (1989). The axis of rotation of the ankle joint. J. Bone Joint Surg. 71-B:94 –99. Michelson, JD., Schmidt, GR., and Mizel, MS., (2000). Kinematics of a total arthroplasty of the ankle: comparison to normal ankle motion. Foot Ankle Int. 21(4):278 –284. Raikin, SM., Heim, CS., Plaxton, NA., and Greenwald, AS., (2000). Mobility Characteristics of Total Ankle Replacements. Cleveland, OH: Orthopaedic Research Laboratories; 2000. Reiley, MA. (2008). INBONE Total Ankle Replacement. Foot Ankle Spec; 1; 305-310 Saltzman, CL., (2000). Perspective on total ankle replacement. Foot Ankle Clin. North Am. 5:761 –775. Saltzman, CL., McIff, TE., Buckwalker, JA., and Brown, TD., (2000). Total ankle replacement revisited. J Orthop Sports Phys Ther.;30:56-67 Spears, I.R., Miller-Young, J.E., Sharma, J., Ker, R.F., Smith, F.W. (2007) ‘The potential influence of the heel counter on internal stress during static standing: A combined finite element and positional MRI investigation’. J Biomech. Spears, I.R., Miller-Young, J.E., Waters, M., Rome, K. (2005) ‘The effect of loading conditions on stress in the barefooted heel pad’. Med Sci Sports Exerc, 37:1030–36. Wood, PLR., (2003). Which ankle prosthesis? Presented at the 29th Annual Meeting of the British Orthopaedic Foot Surgery Society, Cumbria, UK, May 1–3, 2003. Read More