Weight-bearing imaging to assess pathology has become the standard of care in foot and ankle orthopaedics.1,2 Weight bearing better represents and reveals underlying pathologies such as malalignment, impingement, joint space narrowing, and instability that may not be fully appreciated when the foot is offloaded.1 Until recently, radiographs were the only readily accessible imaging modality that could be done with the patient fully bearing weight.1,3 The advent of cone-beam CT technology rather than conventional multidetector CT configurations allowed the detector to move around the patient without the patient moving through the scanner and made the development of the weight-bearing CT (WBCT) scanner possible.3
Standard weight-bearing radiographs are limited by projection, foot orientation, and difficulty in assessing the three-dimensional relationship of structures because of the bones being superimposed.2 In addition, measurements on radiographs require calibration. CT scans overcome these deficits by producing three-dimensional images that do not require calibration. Measurements of joint alignment and angles on cone-beam WBCT scans have been shown to be more accurate than on conventional radiographs and nonweight-bearing CT scans, and this may have clinical implications for surgeons planning deformity correction before surgery.2 As an example, Hirschmann et al4 reported notable differences when comparing measurements of impingement (eg, fibulocalcaneal distance) and joint space width (eg, lateral talocalcaneal joint space width) between weight-bearing and nonweight-bearing CT scans in 22 patients (Figure 1, A and B).
WBCT scans have several other advantages over conventional imaging modalities. Radiation exposure in cone-beam WBCT scans has been estimated to be approximately 10% to 66% less than conventional multidetector CT scanners.1,3,5 Less ionizing radiation exposure is a notable advantage of cone-beam WBCT scans over conventional CT scans and may be advantageous for those patients who require frequent imaging or have complex deformities. One recent study also demonstrated the cost effectiveness and decreased image acquisition time of WBCT scanners over conventional radiographic and CT imaging modalities.5 For centers that do not have access to WBCT scans at this time, a traditional CT scanner with an axial loading device that is able to produce greater than 70% of the patient's body weight has been shown to accurately represent full weight-bearing and can be used as a substitute for newer three-dimensional standing WBCT scanners.6 However, such devices still do not achieve full weight-bearing and do not truthfully represent the foot in a standing (“stance”) position.
These advantages of WBCT scans, however, must be weighed against their higher cost and increased ionizing radiation exposure to patients over standard weight-bearing radiographs. Weight-bearing radiographs remain sufficient to adequately diagnose and treat most foot and ankle pathologies, and many of the studies included in this review do not directly compare WBCT scans with weight-bearing radiographs. Therefore, the superiority of WBCT scans over traditional weight-bearing radiographs in many foot and ankle diagnoses has not been established.
The purpose of this review article on WBCT scans in foot and ankle orthopaedics is to summarize the current literature, describe how WBCT scans have been used for research purposes to help better understand foot and ankle pathologies, and demonstrate how this technology may be applied to clinical practice.
Normal Anatomy on Weight-bearing CT Scans
WBCT scans have been used to investigate normal foot and ankle anatomy in patients without notable pathology.7-11 In healthy control patients, WBCT has been most frequently used to study hindfoot alignment, which has important implications for load-bearing through the lower extremity.8,11,12 A recent study that reviewed the WBCT scans of 48 patients without hindfoot pathology described a neutral hindfoot alignment in this control subject cohort rather than the presence of innate valgus of the hindfoot, which had been previously suggested based on weight-bearing radiographs.9 Others have taken advantage of a newer measurement called the foot and ankle offset (FAO), which is a three-dimensional measurement that defines a relationship between three points on the sole of the foot and one in the center of the ankle joint, to describe the overall hindfoot alignment.8,11 In these studies, they found that the hindfoot alignment in normal patients lies slightly valgus to the midline.8,11 These studies describing hindfoot alignment in normal control patients may provide surgeons with a reference when planning for deformity correction.
Another study used a WBCT scanner with a custom pedography sensor to compare the morphologic center of the foot with the center of force during weight-bearing.10 They found that the morphologic center of the foot was distal to the center of gravity or force of the foot in 97% of feet at a mean distance of 27.5 mm distal.10 In 62% of feet, the foot center was lateral to the center of gravity but only by a mean of 2.0 mm.10
In addition to investigating alignment and force distribution through the foot and ankle, WBCT scans have been used in normal patients to describe subtalar joint position and rotational dynamics of the talus because these are difficult to visualize well on plain radiographs.13 A study by Colin et al13 in 59 patients without hindfoot pathology demonstrated that the most posterior aspect of the posterior facet of the subtalar joint is consistently oriented in valgus, whereas the most anterior aspect of the posterior facet of the subtalar joint is typically oriented in varus. This study suggests that the measurements of the varus-valgus orientation of the posterior facet of the subtalar joint are dependent on where the CT image is taken in the AP direction. Lepojärvi et al14, using 32 healthy control subjects, showed that the talus rotates a total of 10° internally-externally about the tibia when the ankle is taken from maximal internal to maximal external rotation without substantial widening of the medial clear space.
Weight-bearing CT Scans in Adult-Acquired Flatfoot Deformity
Adult-acquired flatfoot deformity (AAFD) encompasses a number of pathologies including medial longitudinal arch collapse, hindfoot valgus, midfoot abduction, and compensatory forefoot varus. These concomitant deformities can be difficult to evaluate individually on two-dimensional weight-bearing radiographs, and visualization of the subtalar joint and impingement between the calcaneus and fibula are better assessed on WBCT scans. Ferri et al15 found that CT scan measurements of forefoot arch angle, which is the angle between the horizontal and a line connecting the inferior aspect of the medial cuneiform to the inferior aspect of the fifth metatarsal, and subtalar joint subluxation were markedly changed when applying 50% weight-bearing compared with a nonweight-bearing state in a simulated weight-bearing study. Another study by Kido et al16 also reported the differences in the talar, navicular, and calcaneal positions on WBCT scans compared with nonweight-bearing CT scans. Consequently, WBCT scans have been used to quantify the severity of deformities in AAFD with good intraobserver and interobserver reliability.17 In one study comparing 10 control patients with 23 patients with stage II to IV AAFD, WBCT scans demonstrated more notable differences in radiographic parameters between control subjects and patients with AAFD than plain weight-bearing radiographs alone, although the clinical implications of these findings have not been established.18
Subtalar Joint Alignment
An early study using simulated WBCT scans demonstrated frequent subluxation of the talocalcaneal joint with an average of only 68% of the posterior facet of the calcaneus in contact with the talus in eight patients with AAFD compared with 92% in control patients.19 In a retrospective study of 22 feet in 20 patients with symptomatic AAFD who had undergone simulated WBCT scans, the anterior aspect of the posterior facet was typically oriented in valgus in contrast to a comparison cohort of normal control patients who had a varus orientation of the anterior aspect of the posterior facet (Figure 2, A and B).20 This varus orientation at the anterior part of the posterior facet of the subtalar joint in normal control patients was consistent to the results reported by Colin et al,13 who used upright, full WBCT scans, previously discussed. Similarly, the posterior aspect of the posterior facet of the subtalar joint in patients with AAFD was found to be in statistically significantly more valgus in patients with flatfoot than in normal control subjects.20 More recent studies have used standing, full WBCT scans, in contrast to simulated WBCT scans, to investigate the orientation of the subtalar joint in patients with stage II AAFD compared with control patients.21 Cody et al,21 using full WBCT scans, measured the subtalar joint alignment at 50% of the anterior-to-posterior dimension of the posterior facet in 45 patients with stage II AAFD and in 17 control patients. They found that stage II patients with AAFD had an additional 10° of subtalar valgus compared with the control subjects.21 A similar study examining subtalar joint valgus in stage II patients with AAFD found that nonweight-bearing CTs underestimated the true subtalar joint deformity compared with WBCT scans and could not be used as a substitute for full WBCT scans.22
Subtalar and Subfibular Impingement
Patients with AAFD may present with pain laterally at the anterior fibula and sinus tarsi due to calcaneofibular (subfibular) and/or talocalcaneal (subtalar) impingement (Figure 3, A and B).23,24 Malicky et al23 used simulated WBCT scans in 19 patients with AAFD who were indicated for surgery to study these impingement deformities. They found evidence of subtalar impingement in 92% of the WBCT scans and observed subfibular impingement in 66% of the CT scans, which differed markedly from control patients who had rates of 5% and 0%, respectively.23 MRI, although nonweight-bearing, may also be used to evaluate the subtalar joint for evidence of subtalar impingement and to measure the calcaneofibular distance for evidence of subfibular impingement.
More recently, Jeng et al24 used WBCT scans in 25 patients with AAFD to evaluate subtalar and subfibular impingement. Thirteen patients had stage II AAFD, 17 had stage III, one had stage IV, and three patients were not able to be staged based on chart notes.24 In contrast to Malicky et al, only 38% of patients had subtalar impingement and only 35% of patients had subfibular impingement on WBCT scans.24 Patients with subfibular impingement had a mean coronal calcaneofibular distance of 2.7 mm compared with those patients without subfibular impingement who had an average coronal calcaneofibular distance of 5.7 mm.24 In addition, they found that patients with AAFD with either talocalcaneal or calcaneofibular impingement had, on average, approximately 10° more of talonavicular abduction than patients with AAFD without bony impingement. These findings may have important implications for predicting which patients may fail a flatfoot reconstruction.
WBCT has also been shown to aid in the assessment of hindfoot deformity in patients with AAFD, although no direct comparisons have been made between WBCT scans and weight-bearing radiographs. In a study of 20 patients with stage II AAFD, de Cesar Netto et al25 reported that clinical examination of the hindfoot alignment angle underestimated the hindfoot alignment angle as measured on WBCT scans by a mean of 7.6°. They reported that the mean hindfoot moment arm measured on WBCT scans was 15.1 mm in valgus, which correlated markedly with the clinical hindfoot alignment angle.25 They advocated for the use of WBCT scans in patients with AAFD because they found the measurements of hindfoot alignment to be repeatable and reliable.25 Burssens et al were the first to describe the hindfoot alignment angle on WBCT scans, which was similar to the technique used by de Cesar Netto et al mentioned above, and found that patients with AAFD had an average hindfoot alignment angle of 20.1°.12 Such measurements have the potential to help surgeons titrate the amount of correction of various osteotomies.
Medial Longitudinal Arch Alignment and Forefoot Varus
To investigate the alignment of the bones along the medial longitudinal arch in AAFD, Kido et al16 used simulated WBCT scans to study 24 patients with flatfoot deformity and compared them with 20 healthy control subject feet. When the foot was loaded, patients with AAFD had more dorsiflexion at the first tarsometatarsal joint and more eversion at the talonavicular and talocalcaneal joints than the healthy control subjects.16 Their findings were supported by those of Zhang et al26 who compared similar WBCT measurements in patients with stage II AAFD to healthy control patients. Greisberg et al27 also used simulated WBCT scans in 37 patients with AAFD to study the medial longitudinal arch. Deformity along the medial longitudinal arch was found to be at either the navicular-cuneiform joint (65%) or talonavicular joint (20%) but rarely at both, with only one foot having more than 10° of collapse at both joints.27 They also noted severe degenerative changes in 38% of patients at the talonavicular joint, but none of the tarsometatarsal joints showed notable arthritic changes on the CT scans.27
Yoshioka et al28 used simulated WBCT scans to study forefoot alignment in patients with stage II AAFD. They reported that the fifth metatarsal bone was more plantarflexed in patients with AAFD than in healthy control patients, and the authors described this finding as the compensatory forefoot varus deformity.28 However, it should be noted that the difference in plantar flexion of the fifth metatarsal bone between flatfoot patients and normal control subjects was only 1.3°.28 This could have important implications in the surgical management of the compensatory forefoot varus pathology and may suggest that the deformity is prone to overcorrection.
Weight-bearing CT Scans in Hallux Valgus
Pronation of the First Metatarsal
Although hallux valgus (HV) has been understood to be a triplanar deformity of the first metatarsal, WBCT has recently helped to quantify the pathology at the first tarsometatarsal joint as well as the rotational deformity of the first ray. Collan et al29 used WBCT scans to compare 10 patients with HV with five asymptomatic normal control patients. They found no difference in the sagittal first metatarsal-ground angle between the HV and normal control subject groups. To study rotational deformities, they used a two-dimensional coronal view and measured the angle between the ground and either the sesamoid articulation of the first metatarsal or base of the proximal phalanx. The authors reported no notable difference in pronation of the first metatarsal in patients with HV (mean, 8°) and normal (mean, 2°) patients; however, they did note a statistically significant difference in the pronation of the proximal phalanx between the HV (mean, 33°) and control subject (mean, 4°) groups.29 The lack of statistical significance in pronation of the first metatarsal between HV and control patients may be due to the small number of patients included in the study and the authors' technique used to measure pronation based on the plantar aspect of the first metatarsal head, which may be deformed due to subluxation of the sesamoids and erosion of the crista.
In contrast, more recent studies have found increased pronation of the first metatarsal in patients with HV compared with normal control subjects.30,31 Kim et al30 demonstrated an increase of 8.1° in pronation of the first ray in patients with HV compared with control patients. Campbell et al31 used a computer-aided design to create a three-dimensional model of the first metatarsal and then measured the pronation of the first ray with the second metatarsal as a reference because the second ray forms a central, structural element of the foot (Figure 4). Using simulated WBCT scans, the authors reported that patients with HV had 9.9° of increased pronation of the first metatarsal compared with normal control subjects.31 In addition, they did not find any correlation between pronation of the first ray and IMA or HVA in HV.31 A subsequent study using the technique developed by Campbell et al found that the average preoperative pronation of the first metatarsal in patients with HV was 29.0° and decreased to 20.2° postoperatively, which was statistically significant.32 Pronation of the first ray was not associated with sesamoid position, suggesting that the position of the sesamoids is likely not a good proxy for first metatarsal pronation.32 Consequently, the rotational deformity is likely independent of the other deformities in patients with HV. Proper recognition and correction of the pronation deformity of the first ray may be important in the surgical management of patients HV.
Two studies using simulated WBCT scans have demonstrated increased mobility at the midfoot in patients with HV when compared with normal patients.33,34 Kimura et al33 performed simulated WBCT scans in 10 patients with HV and 10 patients without HV to investigate motion at the midfoot. In the HV group, they demonstrated statistically significantly greater dorsiflexion, inversion (pronation), and adduction of the first metatarsal in relation to the medial cuneiform compared with the control patients, which suggested increased motion of the first ray in patients with HV.33 In a separate study using simulated WBCT scans, the same group found greater mobility of the first-second intercuneiform joint in patients with HV.34 Another study compared simulated weight-bearing and nonweight-bearing CT scans in 10 healthy control patients and 10 patients with HV.35 A three-dimensional model of each bone was created so that widening and translation of the first metatarsal-cuneiform joint could be measured.35 In patients with HV, the first metatarsal-cuneiform joint significantly widened and translated more in the dorsal-plantar direction when compared with the normal control patients, which supports the finding of hypermobility of the first tarsometatarsal joint in patients with HV.35
Position of the sesamoids may also be more accurately measured on WBCT scans.30 Kim et al30 used simulated WBCT scans to demonstrate that no correlation was found between sesamoid position and pronation of the first metatarsal, which suggests that the degree of subluxation of the sesamoids is independent of first ray pronation. In approximately 25% of cases, they found that patients had “pseudosesamoid subluxation” on simulated WBCT scans in which there was first metatarsal pronation without true sesamoid subluxation; however, when viewed on an AP weight-bearing radiograph, the sesamoids in these cases would appear subluxated.30 Another simulated WBCT study demonstrated that tibial sesamoid position was correlated with HV severity as measured by the HV and intermetatarsal angles.36 They also found that the degree of degenerative change in the sesamoid metatarsal joint was associated with increasing lateral shift of the tibial sesamoid (Figure 5).36
Weight-bearing CT Scans to Evaluate the Syndesmosis
WBCT scans have also been used to examine motion and investigate biomechanics at the distal tibiofibular syndesmosis in both injured and uninjured patients.7,37-40 Motion at the syndesmosis under weight-bearing conditions was quantified in one study on 32 healthy control patients.7 Using WBCT scans, they found that the fibula is located anterior in the tibial incisura in 88% of patients.7 Physiologic motion of the incisura also occurs as the fibula moves, on average, 1.5 mm in the anterior-posterior direction and 3° in external rotation as the foot is moved from maximal internal to external rotation.7 Another retrospective study found similar results in a group of 26 patients who had undergone both nonweight-bearing and weight-bearing CT scans with foot and ankle diagnoses that were not believed to affect the syndesmosis.37 They found that the fibula externally rotates and translates posteriorly and laterally during weight-bearing (Figure 6).37
In patients with syndesmotic injuries, there is high interobserver and intraobserver reliability of distal tibiofibular WBCT scan measurements.40 However, the utility of WBCT scans in the diagnosis of syndesmotic injury is unclear. Burssens et al38 compared WBCT scan measurements in 12 patients with syndesmotic injuries and seven normal control patients. In the patients with syndesmotic injuries, they found increased movement of 1.4 mm in mediolateral direction of the lateral malleolus in the incisura.38 They also reported increased external rotation by 4.5° in the injured cohort.38 In contrast, Hamard et al39 found that WBCT scans were less effective at distinguishing pathologic syndesmotic injury than conventional nonweight-bearing multiplanar CT scans. They used nonweight-bearing multiplanar CT scans to analyze 11 ankles with suspected syndesmotic injury and used WBCT scans to investigate an additional of eight ankles with suspected syndesmotic injury.39 True syndesmotic instability in all ankles was determined using ankle arthroscopy.39 The results of this study suggested that conventional nonweight-bearing multiplanar CT scans were more accurate in determining syndesmotic injury.39 The authors hypothesized that physiologic widening of the distal tibiofibular syndesmosis during weight-bearing in the upright position may account for these results.39 Consequently, at this time, WBCT scans have been more important in advancing the understanding of physiologic motion at the syndesmosis than in the diagnosis of syndesmotic instability. Future work could help to identify appropriate reduction of the syndesmosis after injury.
Weight-bearing CT Scans to Evaluate Lateral Ankle Instability
Ankle sprains are commonly encountered in foot and ankle orthopaedic practices and are typically managed nonoperatively. However, a subset of patients develop chronic lateral ankle instability, which, if left untreated, can cause recurrent osteochondral injury and ultimately tibiotalar arthritis. WBCT scans have been used to study the alignment of the foot and ankle in patients with chronic lateral ankle instability to determine if certain risk factors can be elucidated.
In an early study, van Bergeyk et al41 used simulated WBCT scans to study hindfoot alignment in 12 patients with three or more episodes of lateral ankle sprains or instability with ongoing symptoms for at least 6 months. They compared this cohort of patients with 12 control patients.41 The study demonstrated that patients with chronic lateral ankle instability had measurements of hindfoot alignment including calcaneal metatarsal angle and medial calcaneal varus angle that were, on average, in approximately 3° to 4° more varus than the control patients.41 The authors concluded that hindfoot varus was an important factor correlated with recurrent ankle instability.41
More recently, Lintz et al42 used measurements on WBCT scans to examine hindfoot alignment in patients with chronic lateral ankle instability. They compared FAO, calcaneal offset, and the hindfoot angle in 34 patients with chronic lateral ankle instability with 155 patients without ankle instability.42 Calcaneal offset represents the difference (in millimeters) between a neutral position of the calcaneus and the actual position of the calcaneus.42 The hindfoot angle is formed between the weight-bearing axis of the tibia at the apex of the center of the talar dome and a line draw through the long axis of the calcaneus.42 Patients with chronic lateral ankle instability had approximately 4% more varus in their FAO, 9 mm more of varus in their calcaneal offset, and 16° more of varus in their hindfoot angle.42 For every 1% change in the FAO toward a varus alignment, there was a 35% increased odds ratio of developing chronic lateral ankle instability.42
Weight-bearing CT Scans in Clinical Practice
In addition to helping surgeons understand foot and ankle biomechanics and pathologies, WBCT scans are increasingly changing clinical practice by allowing surgeons to better assess deformities and preoperatively plan for surgical intervention. At this time, WBCT scans may be especially valuable to the clinician in assessing deformities such as subtalar and subfibular impingement in AAFD, hindfoot alignment, and pronation of the first metatarsal in HV.
In AAFD, surgeons may use WBCT scans to look for subtalar impingement at the angle of Gissane on sagittal views or subfibular impingement on coronal views. Patients with severe subtalar or subfibular impingement may require a subtalar arthrodesis to properly align the foot and prevent future impingement or recurrence because current techniques such as a medializing calcaneal osteotomy and lateral column lengthening may not address these deformities. Future work may identify patients who are likely to fail reconstructive procedures.
WBCT scans could also be used clinically to better evaluate hindfoot alignment. This may be particularly helpful in AAFD and in cases of chronic lateral ankle instability. Subtle varus and valgus deformities may be difficult to quantify on physical examination and on radiographs. WBCT scans may be an important tool to assist surgeons in titrating correction based on an individual patient's specific deformity.
WBCT scans can also be used in practice to determine pronation and instability of the first metatarsal in patients with HV. Because sesamoid position and first ray pronation are distinct deformities, the rotational deformity of the first metatarsal cannot be determined using sesamoid position on weight-bearing radiographs as a proxy for metatarsal pronation. Using WBCT scans to preoperatively understand the amount of pronation of first ray may help surgeons calibrate the appropriate surgical correction of the rotational deformity. In addition, surgeons may choose to use WBCT scans in patients with HV to evaluate for instability at the first tarsometatarsal (TMT) joint, which could assist in identifying which patients require a first TMT arthrodesis.
Finally, there may be a role for WBCT scans in the creation of patient-specific instrumentation, although there is no evidence at this time that demonstrates an advantage of WBCT scans for this use. WBCT scans may be used to more accurately model a patient's anatomy and create three-dimensional implants or cutting guides to help surgeons better treat complex deformities.
The advent of cone-beam CT technology in place of conventional multidetector CT scanner configurations has made the development of WBCT scans possible. Cone-beam CT technology also has the advantage of reducing ionizing radiation exposure to the patient. It has two-thirds the effective radiation dose of a conventional CT scan but approximately 2.5 times as much radiation as a standard, three-view weight-bearing radiograph of the foot.18 Compared with conventional nonweight-bearing CT scans, WBCT scans better demonstrate the true orientation of the bones and joints during loading conditions and help to identify underlying pathologies such as malalignment, impingement, and instability. They have provided new insight into common foot and ankle disorders such as AAFD, HV, ankle fractures, and lateral ankle instability. WBCT scans, however, have not replaced lower cost weight-bearing radiographs, which are often sufficient to adequately diagnose and manage most foot and ankle pathologies. At this time, WBCT scans may be better used as an adjunct. In clinical practice, WBCT scans may help surgeons assess subtalar and subfibular impingement and hindfoot alignment in AAFD. This technology may also help evaluate rotational deformities such as pronation of the first metatarsal in patients with HV. As WBCT scanners become more frequently used to evaluate foot and ankle pathologies, additional indications will soon emerge. Understanding the application of WBCT scans to clinical practice is becoming increasingly important for surgeons as they strive for better outcomes in the management of complex foot and ankle disorders.
References printed in bold type are those published within the past 5 years.
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