Abstract
Background
Minimally invasive (MIS) treatment of hallux valgus (HV) deformity is increasing in popularity. A 2-mm diameter burr is used to create a distal first metatarsal osteotomy prior to capital fragment translation and fixation. The metatarsal will shorten by the burr’s diameter (2 mm). Plantar or dorsal capital fragment displacement may also cause load transference and possibly transfer metatarsalgia. The purpose of this study is to examine the effect of MIS HV on forefoot loading mechanics with respect to metatarsal shortening and sagittal plane displacement.
Methods
Four lower-limb cadaveric specimens were studied. A pedobarography pressure-sensing mat was used to record forefoot plantar pressure in a controlled weight-bearing stance position. Control and postosteotomy measurements were obtained with the capital fragment fixated in 3 possible positions: 0 mm, 5 mm dorsal, and 5 mm plantar displacement. Pedobarography data yielded pressure data within measurable graphical depictions. Raw mean contact pressure measurements were taken under the first and fourth metatarsal heads to establish medial and lateral forefoot loading pressure ratios. An a priori power analysis was performed based on previous peer-reviewed pedobarographic data, and our study was adequately powered.
Results
Around 40 measurements were recorded, and ratios of medial-to-lateral forefoot loading were constructed. Medial forefoot pressure control versus 0 mm displacement, and control versus dorsal displacement were not found to be statistically significant (p = 0.525, p = 0.55, respectively). Medial pressure significantly increased when comparing control versus plantar displacement (P = .006). Lateral pressure significantly increased with dorsal displacement of the osteotomy (P = .013).
Conclusion
Our study found that MIS HV correction did not cause an increase in lateral forefoot pressure loading when sagittal plane displacements were controlled. Plantar displacement increased medial loading, and dorsal displacement increased lateral loading. It may be valuable for surgeons to consider metatarsal head position postosteotomy, as a decrease in medial loading and subsequent increase in lateral loading may lead to lateral forefoot pain and transfer metatarsalgia.
Levels of Evidence:
IV.
“Dorsal displacement of the osteotomy did increase lateral forefoot pressure which may result in transfer metatarsalgia”
Introduction
Hallux valgus (HV) deformity is a common pathology treated by foot and ankle surgeons. Many surgical techniques to treat HV have been described in the orthopaedic literature, and there has been a recent trend of surgical treatment utilizing minimally invasive (MIS) techniques. Minimally invasive technique has been cited to achieve faster recovery, a smaller surgical scar, and higher patient satisfaction when compared to the traditional, open approach for HV correction. 1 However, this procedure requires specialized training with the use of the burr and fluoroscopy. This may result in a steep learning curve estimated to be 20 to 50 cases, which may contribute to postoperative complications.2-4
Control of the capital fragment and visualization of its alignment on fluoroscopy is a known challenge with this procedure and the literature has noted associated complications.5,6 Third-generation MIS for HV uses a low-speed, high-torque Shannon burr to create a distal first metatarsal osteotomy. The capital fragment is then laterally translated and supinated to correct the HV deformity. Fixation is then commonly achieved using a percutaneous screw construct.
Minimally invasive technique correction of HV may result in alteration of forefoot pressure distribution with weight bearing via 2 primary mechanisms. The burr has a kerf, or cutting diameter, of 2 mm, which is thicker than that of the oscillating saw used in open techniques and may lead to shortening of the first metatarsal. Shortening of the first metatarsal has been shown to reduce plantar pressure beneath the first metatarsal head and increase pressure beneath the lateral (second to fourth) metatarsal heads.7,8 This alteration in pressure may lead to clinically relevant transfer metatarsalgia, which has been documented to occur in 5.6% of MIS for HV cases. 9
Transverse percutaneous distal metatarsal osteotomy has been described to achieve better rotational correction compared to the chevron approach. 10 The purpose of this study is to determine whether forefoot pressure is affected by the metatarsal shortening and sagittal plane displacement of transverse MIS osteotomy for the treatment of HV.
Methods
Four fresh, isolated lower limb cadaveric specimens were used in this study. No specimens had prior implanted hardware in the foot or ankle, arthritic changes, or other sequelae of prior surgery or injury. The specimens were not found to have clinical or radiographic HV deformity. Three of the specimens were right lower limbs and one was a left lower limb.
A pedobarography pressure-sensing mat (MobileMat, TekScan, Norwood, Massachusetts) was used to record the plantar pressures of each specimen, which was similar to prior reports. 11 Each measurement was controlled to 100 lbs of stance-phase downward force using a digital scale with the mat. All measurements were repeated for measurement consistency observation.
Pedobarography measurements were taken first as controls prior to osteotomy using manufacturer measurement software (FootMat Research 7.10, TekScan). Next, a transverse distal first metatarsal osteotomy was performed with a 2 mm × 20 mm Shannon burr. An open dorsomedial approach to the first metatarsal was used to confirm sagittal plane displacement utilizing direct visualization and a ruler. The capital fragment was then rigidly fixated using 2 Kirschner wires in 3 positions: (1) Zero mm sagittal displacement of the distal fragment (matching dorsomedial cortex, neutral position), (2) 5 mm of plantar MT head displacement, and (3) 5 mm of dorsal MT head displacement. The neutral position was then repeated for consistency measurements. Pedobaorgraphy measurements were recorded twice in each position to measure consistency across readings. Pedobarography data yielded peak and mean contact pressure as well as contact pressure area in a graphical depiction. Raw measurements were taken under the first and fourth metatarsal heads to establish medial and lateral forefoot pressure measurements, respectively. A ratio of medial to lateral pressure and contact pressure area was then created for each option in each specimen to portray changes in forefoot weight-bearing.
An a priori power analysis was performed based on previous peer-reviewed pedobarographic data demonstrating clinically relevant pressure changes for percutaneous distal osteotomy of the lesser metatarsals. 12 Power analysis was performed, which required 10 measurements. Observer consistency measurements were recorded. Effect sizes were then calculated for clinical and practical utility.13,14
Results
Forty measurements were recorded. Mean contact forefoot pressures and areas for the preosteotomy control versus 0 mm displacement, 5 mm dorsal displacement, and 5 mm plantar displacement, respectively, are documented in Tables 1 and 2. Medial forefoot pressure did not significantly increase from the preosteotomy control position to postosteotomy with 0 mm displacement (medial/lateral forefoot pressure ratio: 1.03-1.38 respectively, P = .525) and did not significantly decrease with dorsal translation (medial/lateral forefoot pressure ratio: 1.03-0.867, respectively, P = .55) (Table 3).
Mean Contact Pressure and Mean Contact Area for Preosteotomy Control Group vs Postosteotomy With 0 mm of Sagittal Displacement.
Mean Contact Pressure and Mean Contact Area For Postosteotomy With 5 mm of Plantar Displacement and Postosteotomy With 5 mm of Dorsal Displacement Respectively.
Medial to Lateral Pressure Ratios Between Different Levels of Sagittal Metatarsal Displacement of the Osteotomy.
Bold values indicating statistical significance.
Based off medial/lateral pressure ratios, a significant increase in medial pressure was identified when comparing control (1.03) versus plantar (2.12) displacements (P = .006). Lateral pressure was found to increase for plantar (0.867) versus dorsal (2.12) displacement of the osteotomy (P = .013) (Table 3). We examined measurement data consistency for each specimen utilizing the coefficient of variation (CV, Equation 1), and the average CV was found to be 0.18 (range: 0.07-0.33). Coefficient of variation <0.3 is generally accepted to represent low variation and dispersion around the mean, indicating an acceptable measurement technique.
Using the contact pressure means and standard deviations, the effect size of preosteotomy versus postosteotomy with 0 mm displacement was 1.49 (Cohen’s d). The effect size of preosteotomy versus postosteotomy with 5 mm dorsal displacement was 0.68 (Cohen’s d). The effect size of preosteotomy versus postosteotomy with 5 mm plantar displacement was 1.69 (Cohen’s d).
Discussion
Minimally invasive technique for the treatment of HV deformity has recently increased in utilization, but some concern remains regarding associated complications such as transfer metatarsalgia. The results of this study demonstrated that, when performing MIS transverse distal metatarsal osteotomy, plantar displacement of the osteotomy increased medial forefoot pressure but did not appear to increase lateral forefoot pressure which can be associated with transfer metatarsalgia. Conversely, the results showed that dorsal displacement of the osteotomy did increase lateral forefoot pressure which may result in transfer metatarsalgia. Furthermore, shortening of the metatarsal caused by the kerf of the burr with neutral translation did not increase medial or lateral forefoot pressures.
Hallux valgus deformity is associated with altered forefoot loading, insufficient medial forefoot loading, and lateral weight transference. 15 There is no established “normal” value of medial forefoot pressure; however, patients with HV may already have a relative increase in lateral forefoot pressure and loading that surgeons may seek to counteract. The results of this study demonstrated that surgeons may consider avoiding dorsal translation of the capital fragment for patients with HV since this has the potential to increase lateral forefoot loading and therefore induce or exacerbate transfer metatarsalgia. A concern that has been raised with the use of MIS technique for HV surgery is the effects of metatarsal shortening caused by the kerf of the burr when performing the osteotomy. In this study, MIS transverse distal metatarsal osteotomy with a neutral sagittal alignment did not increase lateral weight transfer despite shortening the first ray by 2 mm.
The results of this study demonstrated that comparing the preosteotomy control group with postosteotomy with 0 mm displacement and 5 mm plantar displacement, respectively, yielded very large effect sizes. This indicates that there was a 71% to 81% of nonoverlap for these measurements. 16 Comparing the preosteotomy control group with postosteotomy with 5 mm dorsal displacement yielded an effect size between medium and large, indicating a 33% to 47% of nonoverlap. Given this, according to Cohen, a medium effect of .5 (Cohen’s d) is visible to the naked eye of a careful observer. All effect sizes in this study exceeded this value and therefore indicated practical and clinical significance that may be valuable for surgeons to consider. 17 Of note, this pilot study is based on prior related work and may indicate that further investigation is warranted with a larger sample size and human subjects.
This study has inherent limitations like any controlled laboratory biomechanical cadaveric evaluation which may not match in vivo forefoot loading conditions. First, our study only included 4 cadavers, and more specimens would have increased the generalizability of our results. In addition, measurements were performed in a static stance position on a pedobarography measurement mat as opposed to a dynamic gait evaluation. Also, the specimens in our study did not have HV deformity. It is known that patients with HV have forefoot loading alterations and thus baseline, preosteotomy forefoot pressures may not reflect the HV foot. Finally, although pedobarography measurement data was consistent in our experiment, this may not match in vivo forefoot loading and further investigation may be indicated to determine whether sagittal head translation after MIS for HV correction can cause lateral load transfer and clinical metatarsalgia.
Conclusion
This study found that MIS for HV correction with transverse distal metatarsal osteotomy did not appear to cause an increase in lateral forefoot pressure loading when sagittal plane displacements were controlled. However, plantar displacement increased medial forefoot loading and dorsal displacement increased lateral forefoot loading. It may be valuable for surgeons to consider metatarsal head position postosteotomy, as a decrease in medial loading and subsequent increase in lateral loading may lead to lateral forefoot pain and transfer metatarsalgia, which is particularly problematic for patients with HV.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
Ethics Approval
This study was IRB exempt as it was a cadaver study.
Informed Consent
This study was IRB exempt and was a cadaveric study.
Trial Registration
Not a RCT.
