Fig. 2.1
In a foot with a normally positioned subtalar joint (STJ) axis (center), the ground reaction force plantar to the calcaneus (GRFC), will cause a STJ supination moment since it acts medial to the STJ axis. Ground reaction force acting plantar to the fifth metatarsal head (GRFFF) will cause a STJ pronation moment since it acts lateral to the STJ axis. In a foot with a medially deviated STJ axis (left), since the plantar calcaneus now has a decreased STJ supination moment arm when compared to normal, GRFC will cause a decreased magnitude of STJ supination moment. Since the fifth metatarsal head has an increased STJ pronation moment arm, GRFFF will cause an increased magnitude of STJ pronation moment when compared to normal. However, in a foot with a laterally deviated STJ axis (right), since the plantar calcaneus now has an increased STJ supination moment arm, GRFC will cause an increased magnitude of STJ supination moment, and since the fifth metatarsal head has a decreased STJ pronation moment arm, GRFFF will cause a decreased magnitude of STJ pronation moment when compared to normal. Therefore, the net result of the mechanical actions of ground reaction force on a foot with a medial deviated STJ axis is to cause increased magnitude of STJ pronation moment, and the net mechanical result of a laterally deviated STJ axis is to cause increased magnitude of STJ supination moment. (Reprinted with permission from Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465–488, 2001)
Fig. 2.2
In the model above, a posterior view of the right foot and ankle are modeled as consisting of the talus and tibia combined together to form the talotibial unit which articulates with the foot at the subtalar joint (STJ) axis. The external forces acting on the foot include ground reaction force (GRF) plantar to the calcaneus (GRFC), GRF plantar to the medial forefoot (GRFM), and GRF plantar to the lateral forefoot (GRFL). In a foot with a normal STJ axis location (center), the more central location of the STJ axis relative to the structures of plantar foot allows GRFC, GRFM, and GRFL to cause a balancing of STJ supination and STJ pronation moments so that more normal foot function occurs. In a foot with a medially deviated STJ axis (left), the more medial location of the STJ axis relative to the plantar structures of the foot will cause a relative lateral shift in GRFC, GRFM, and GRFL, increasing the magnitude of STJ pronation moment and causing more pronation-related symptoms during weight-bearing activities. In a foot with a laterally deviated STJ axis (right), the more lateral location of the STJ axis relative to the plantar structures of the foot will cause a relative medial shift in GRFC, GRFM, and GRFL, increasing the magnitude of STJ supination moment and causing more supination-related symptoms
Fig. 2.3
In a foot with a normal STJ axis location (center), the posterior tibial (PT), anterior tibial (AT), extensor hallucis longus (EHL), and Achilles tendons (TA) will all cause a STJ supination moment when they exert tensile force on their osseous insertion points since they all insert medial to the STJ axis. However, the extensor digitorum longus (EDL), peroneus tertius (TER), and peroneus brevis (PB) tendons will all cause a STJ pronation moment when they exert tensile force on their insertion points since they all insert lateral to the STJ axis. However, in a foot with a medially deviated STJ axis (left), since the muscle tendons located medial to the STJ axis have a reduced STJ supination moment arm, their contractile activity will cause a decreased magnitude of STJ supination moment when compared to normal. In addition, since the muscle tendons lateral to the STJ axis have an increased STJ pronation moment arm, their contractile activity will cause an increased magnitude of STJ pronation moment. In addition, in a foot with a laterally deviated STJ axis (right), since the muscle tendons medial to the STJ axis have an increased STJ supination moment arm, their contractile activity will cause an increased magnitude of STJ supination moment when compared to normal. Since the muscle tendons lateral to the STJ axis have a decreased STJ pronation moment arm, their contractile activity will cause a decreased magnitude of STJ pronation moment. Therefore, the net mechanical effect of medial deviation of the STJ axis on the actions of the extrinsic muscles of the foot is to cause increased magnitudes of STJ pronation moment and the net mechanical effect of lateral deviation of the STJ axis on the actions of the extrinsic muscles of the foot is to cause increased magnitudes of STJ supination moment
In 1992, Kirby and Green first proposed that foot orthoses functioned by altering the external STJ moments that were created by the mechanical actions of ground reaction force (GRF) acting on the plantar foot during weight-bearing activities [93]. They hypothesized that foot orthoses were able to exert their ability to “control pronation” by converting GRF acting lateral to the STJ axis into a more medially located orthosis reaction force (ORF) that would be able to generate increased external STJ supination moments during weight-bearing activities. Using the example of a foot orthosis with a deep inverted heel cup, known as the Blake Inverted Orthosis [21–23, 100], they proposed that the inverted heel cup orthosis produced its impressive clinical results in relieving pronation-related symptoms by increasing the ORF on the medial aspect of the plantar heel so that increased external STJ supination moments would result [93].
Kirby later introduced a foot orthosis modification called the medial heel skive technique (Fig. 2.4) that also produced an inverted heel cup in the orthosis, shifted the ORF medially on the plantar heel, and, as a result, increased the external STJ supination moment to more effectively treat difficult pathologies such as pediatric flatfoot deformity, posterior tibial tendon dysfunction, and sinus tarsi syndrome [24]. The proposed mechanical effect of the medial heel skive modification of shifting the ORF medially on the plantar aspect of the heel of the foot has been supported by recent research by Bonanno et al. [101]. Other similar inverted heel cup modifications to foot orthoses have been introduced since the introduction of the medial heel skive technique which likely mechanically act in a similar manner to the medial heel skive modification [102–104].
Fig. 2.4
In the illustrations above, the posterior aspect of the right foot with a medially deviated subtalar joint (STJ) axis is shown in a shoe without an orthosis (left) and also is shown in a shoe with a medial heel skive foot orthosis (right). In the shoe with only the insole under the foot (left), the medially deviated STJ axis will cause increased STJ pronation moment since the shoe reaction force is more centrally located at the plantar heel. However, when the varus heel cup of a medial heel skive foot orthosis is added to the shoe (right), the resultant medial shift in orthosis reaction force will cause a decrease in STJ pronation moment and an increase in STJ supination moment. Therefore, foot orthoses with varus heel cup modifications, such as the medial heel skive, are more effective at treating symptoms caused by excessive foot pronation due to their ability to shift reaction forces more medially on the plantar foot and, thereby, greatly increase the STJ supination moment acting on the foot
Foot and lower extremity pathologies caused by excessive magnitudes of external STJ supination moment, such as chronic peroneal tendinopathy and chronic inversion ankle sprains, were also proposed to be caused by the interaction of GRF acting on the foot with an abnormally laterally deviated STJ axis [5, 25, 98, 99]. It was suggested that the abnormal STJ supination moments would be best treated with an increased valgus construction within the foot orthosis, including the addition of the lateral heel skive technique [105] within the heel cup of the orthosis. In this fashion, the orthosis would mechanically increase the magnitude of external STJ pronation moments by shifting ORF more laterally on the plantar foot to more effectively treat supination-related symptoms and pathologies.
In the late 1980s and 1990s, a number of other authors likewise started focusing on the idea that orthosis treatment should not be determined by the results of measuring “deformities” of the foot and lower extremity, as proposed by Root and coworkers, but rather should be determined by the location and nature of the internal loading forces and internal stresses acting on and within injured structures of the patient. The idea that pathological internal loading forces acting on the foot and lower extremity in sports and other weight-bearing activities may be effectively modeled to develop better treatment strategies was pioneered by Benno Nigg and coworkers at the University of Calgary, Canada. Nigg and coworkers realized that since invasive internal measurements could not be made on patients to determine the absolute magnitudes of internal loading forces, reliable estimates of these forces could instead be made with more effective models of the foot and lower extremity [106–108].
However, it was not until 1995, when McPoil and Hunt first coined the term “Tissue Stress Model ” that one of the most recent foot orthosis treatment models was given a proper name. McPoil and Hunt suggested that foot orthosis therapy should be directed toward reducing abnormal levels of tissue stress in order to more effectively design mechanical treatment aimed at healing musculoskeletal injuries caused by pathological internal stress acting on and within the structural components of the foot and lower extremity. They felt that by focusing the clinician’s attention on the abnormal stresses causing the injury, rather than on measuring “deformities ” of the lower extremity, that optimal mechanical foot therapy could be better achieved [109].
Following up on the ideas embodied within the Tissue Stress Model , Fuller described, in 1996, how computerized gait evaluation and modeling techniques could be effectively used to guide foot orthosis treatment by aiding in the prediction of abnormal stresses within the foot and lower extremity [110]. Three years later, Fuller described how the location of the CoP on the plantar foot relative to the spatial location of the STJ axis may help direct orthosis therapy for foot pathologies resulting from abnormal STJ moments [111]. In later published works, Fuller and Kirby further explored the idea of reducing pathological tissue stress with orthoses and how this could be integrated with the SALRE Theory of Foot Function and an analysis of midtarsal joint kinetics (Fig. 2.5) to guide the clinician toward a better understanding of foot orthosis function and toward more effective foot orthosis treatments for their patients with mechanically based foot and lower extremity injuries [5, 112, 113]. Recent articles on the shift of foot orthosis treatment paradigms away from the Root model of STJ neutral and toward the Tissue Stress Model of treatment have focused on many of the shortcomings of the Root Subtalar Joint Neutral Model that not only lacks research validation but also uses the unsupported concept that “foot deformities” cause “compensations” or abnormal gait patterns during weight-bearing activities [114, 115].
Fig. 2.5
During standing without a foot orthosis (left), ground reaction force acting plantar to the rearfoot (GRFRF), Achilles tendon tensile force acting on the posterior rearfoot and vertical loading force from the tibia acting onto the superior talus work together to mechanically cause a rearfoot plantarflexion moment which tends to cause the rearfoot to plantarflex at the ankle. In addition, ground reaction force acting plantar to the forefoot (GRFFF) causes a forefoot dorsiflexion moment which tends to cause the forefoot to dorsiflex at the midtarsal joint (MTJ). Both the resultant rearfoot plantarflexion moment and forefoot dorsiflexion moment tend to cause the longitudinal arch of the foot to flatten. However, when a custom foot orthosis is constructed for the foot that applies a significant orthosis reaction force (ORF) to the plantar aspect of the longitudinal arch (right), the resultant increase in ORF at the plantar midfoot combined with the resultant decrease in GRFRF and GRFFF will cause an increase in rearfoot dorsiflexion moment and an increase in forefoot plantarflexion moment. By this mechanical method, foot orthoses help resist longitudinal arch flattening to produce one of the strongest biomechanical and therapeutic effects of orthoses on the foot and lower extremity
In 2001, another new theory of foot orthosis function, the “Preferred Movement Pathway Model ,” was proposed by Nigg and coworkers that was claimed to be a “new paradigm for movement control.” Basing their new theory on previous scientific research, Nigg and coworkers proposed that foot orthoses do not function by realigning the skeleton but rather function by producing a change in the “muscle tuning” of the lower extremity via their alteration of the input signals into the plantar foot during athletic activities. It was suggested that if the preferred movement path is counteracted by the orthosis/shoe combination, then muscle activity would be increased, but conversely, if the preferred movement path is allowed by the orthosis/shoe combination, then lower extremity muscle activity would be reduced [116–118]. Even though the theory of Nigg et al. has received considerable attention within the international biomechanics community , their theory, and all the other abovementioned theories, will require much further research to either support or reject their validity. These and other theories of foot function have been described in much greater detail in the excellent review articles by Payne [119] and Lee [15].
Research on Biomechanical Effects of Foot Orthoses
As mentioned earlier, over the last few decades, there has been a surge in the quality and number of foot orthosis biomechanics research studies on both athletes and nonathletes. Much of the improvement in the quality of research studies on foot orthoses are likely due to many new technological advances that are now available within the modern biomechanics laboratory. These facilities are able to perform advanced biomechanical analyses in a relatively short period of time on subjects using accelerometers, force plates, pressure mats, pressure insoles, strain gauges, and computerized three-dimensional motion analysis. In addition, advanced computer modeling techniques, such as inverse dynamics analysis and finite element analysis, have allowed researchers to better understand the kinetics of gait and investigate the changes in internal loading forces that occur in feet with different orthosis designs. All of these technological advances have allowed researchers to provide very meaningful insights into how foot orthoses biomechanically produce their significant positive therapeutic effects in the treatment of foot and lower extremity injuries [28].
Since early research on the effects of foot orthoses on running biomechanics showed that there was little to no change in the kinematics of gait function with foot orthoses, many doubted whether foot orthoses had any significant biomechanical effect on the foot and lower extremity of the individual [120–123]. However, as the sophistication of biomechanics research has progressed over the past few decades, important new research has now shed more light as to how foot orthoses may change the mechanical function of the foot and lower extremities and help heal injuries in athletes and nonathletes [124–128]. With this newer, more sophisticated research, the multiple alterations that occur in the internal forces and internal moments (i.e., kinetics) of the lower extremities with foot orthoses can now be determined which has produced exciting new research evidence regarding how foot orthoses may produce their biomechanical effects.
Foot Orthoses Alter Foot and Lower Extremity Kinematics and Kinetics
Foot orthoses have been conclusively shown to alter the motion patterns (i.e., kinematics) of the foot and lower extremities in numerous scientific research studies. Research has now shown a decrease in maximum rearfoot eversion angle [120, 121, 128–134], a decrease in maximum rearfoot eversion velocity [121, 128, 133–135], a decrease in maximum ankle dorsiflexion angle [128], a decrease in maximum internal tibial rotation [127, 129, 136, 137], and a decrease in knee adduction [127, 129, 137].
Foot orthoses have also been shown to conclusively alter the internal forces and internal moments (i.e., kinetics) acting on and within the segments of the foot and lower extremity during running. Recent research has shown a decrease in maximum internal ankle inversion moment [126–128, 135] (Fig. 2.6), changes in maximum knee external rotation moment [126], and changes in knee abduction moment [127] during running with foot orthoses. In addition, a decrease in impact peak and maximum vertical loading rate was seen in runners treated with foot orthoses [126].
Fig. 2.6
Research has shown that foot orthoses change the kinetics of gait by altering the internal forces acting on the segments of the foot and lower extremity. In the model illustrated above of the posterior aspect of a right foot with a medially deviated STJ axis, when the posterior tibial muscle contracts with increased force to cause increased tensile force on its tendon, an increased internal inversion moment will be measured (left). However, when an anti-pronation custom foot orthosis is designed for the foot to shift the orthosis reaction force more medial on the plantar heel and longitudinal arch, the resultant increase in external STJ supination moment from the orthosis (see Fig. 2.4) will cause a decrease in posterior tibial muscle contractile force and a decrease in tendon tensile force which will also result in a decrease in measured internal inversion moment (right). It is by this proposed mechanism that foot orthoses may relieve symptoms and heal injuries in the athlete and nonathlete but, in doing so, may also cause little change in measured foot and lower extremity gait kinematics
In addition to the more prevalent research on the biomechanical effects of foot orthoses during running, studies have also shown that foot orthoses significantly affect the biomechanics of walking. Decreased rearfoot pronation and decreased rearfoot pronation velocity with varus-wedged orthoses and increased rearfoot pronation with valgus-wedged were demonstrated in subjects that walked on both varus-wedged and valgus-wedged foot orthoses [133, 134]. In addition, patients with RA that wore foot orthoses for 12 months showed significant reductions in rearfoot eversion and internal tibial rotation [138]. These studies conclusively demonstrate that foot orthoses are able to alter both the motion patterns and internal forces and moments acting within the foot and lower extremity during both running and walking activities. The more recent research on the kinetics and kinematics of foot orthosis function also support the theories mentioned earlier that proposed that foot orthoses work largely by altering the internal forces within the foot and lower extremity by changing the moments acting across the joints of the human locomotor apparatus [5, 25, 93, 99, 106–108, 111–113].
Foot Orthoses Alter Contractile Activity of Lower Extremity Muscles
Research has also shown that foot orthoses significantly affect the contractile activity of the lower extremity muscles during running and other activities. Foot orthoses were found to alter the EMG activity of the biceps femoris and anterior tibial muscles during running [139] and to significantly change the EMG activity of the anterior tibial muscle during walking [140]. Research has shown that changes in foot orthosis design may cause significant changes in EMG activity in many of the muscles of the lower extremity during running [141]. A correlation between perceived foot comfort with different types of foot orthoses and the EMG activity of the lower extremity muscles has also been demonstrated [142]. In addition, in a study of 12 adults with an everted rearfoot posture, foot orthoses were found to significantly decrease the muscular activity of the tibialis anterior, soleus, gastrocnemius, and peroneus longus during walking [143].
Foot Orthoses Improve Postural Stability
As mentioned earlier, there is experimental evidence that foot orthoses can also improve the postural stability of individuals. Postural sway was reduced when subjects wearing foot orthoses were subjected to inversion/eversion and medial/lateral platform movements which indicated that undesirable motion at the foot and ankle may have been restricted and/or the ability of joint mechanoreceptors to detect motion perturbations may have been enhanced by orthoses [85]. Subjects balancing on one foot were likewise shown to have significant decreases in frontal plane CoP length and velocity with medially posted orthoses which possibly indicated foot orthoses enhanced their postural control abilities [86]. In another study involving subjects with excessively pronated feet, foot orthoses produced reductions in medial-lateral sway during bipedal standing indicating improved balance [87].
Foot Orthoses Reduce Plantar Forces and Pressures
Again, as noted earlier, research on the ability of foot orthoses to reduce the forces and pressures on injured or painful areas of the plantar foot provides yet another therapeutic mechanical action of foot orthoses (Fig. 2.7). In a prospective study of 151 subjects with cavus foot deformity, those subjects wearing custom foot orthoses after 3 months showed significant decreases in foot pain, increases in quality of life, and showed three times the forefoot plantar pressure reduction when compared to sham insoles [58]. In 42 subjects with metatarsalgia, foot orthoses were found to not only decrease the metatarsal head pain but also significantly decrease the force impulse and peak pressure at the metatarsal heads [59]. Significant reductions in plantar pressures and loading forces were shown in another study that measured the effects of foot orthoses on both normal and RA subjects [62]. In 81 patients with Type II diabetes, maximum peak plantar pressures were reduced by 30% with foot orthoses [63] and in 34 adolescent Type I diabetic patients both peak pressure and pressure-time integral was reduced while wearing foot orthoses [64]. In a study of eight patients with plantar neuropathic ulcerations that had become healed with custom foot orthoses, it was found that their custom foot orthoses significantly reduced peak vertical pressure, reduced the pressure/time integral, and increased the total contact surface area versus the no-insole condition [61]. In another study using computer-simulated three-dimensional finite element analysis of a foot exposed to different orthosis constructions, orthosis shape was found to be more important in reducing peak plantar pressures than was orthosis stiffness [144].
Fig. 2.7
Research has shown that foot orthoses may be designed to reduce the plantar pressures and forces acting on the foot. In the model above, a frontal plane cross section of the metatarsal heads in a foot with a plantarflexed second metatarsal is illustrated. When the forefoot is close to contacting with the ground, but still is non-weight-bearing, the plantarflexion deformity of the second metatarsal is obvious (left). However, once the forefoot becomes weight-bearing, the increase in ground reaction force (GRF) that occurs at each of the metatarsal heads will be particularly increased at the second metatarsal head (middle) which may cause injuries to the osseous and/or soft tissue structures of the second metatarsal or second metatarsophalangeal joint. To treat the increased compression forces and stresses at the second metatarsal head, a foot orthosis may be designed to increase the GRF plantar to the first, third, fourth, and fifth metatarsal heads and decrease the GRF plantar to the second metatarsal head (right). This redistribution of GRF on the plantar foot, away from high pressure areas toward lower pressure areas, is the most likely mechanism behind the ability of foot orthoses to reduce pathologic pressures away from specific areas of the plantar foot
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