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What The Literature Reveals About IM Angle Correction After First MPJ Arthrodesis
How much reduction of the first intermetatarsal (IM) angle can a surgeon obtain with first MPJ fusion? With this in mind, these authors review the literature and discuss key factors that affect IM angle correction in relation to first MPJ arthrodesis.
A widely accepted treatment, first metatarsophalangeal joint (MPJ) arthrodesis is an extremely versatile procedure that one can utilize in a number of clinical scenarios.1 Not only is first MPJ arthrodesis effective for hallux rigidus, one may also employ this procedure in the treatment of inflammatory arthropathies, subluxations, dislocations, hallux varus and hallux valgus.1-4 When it comes to hallux valgus, the surgical goals for first MPJ fusion include pain relief, deformity reduction and definitive, permanent correction.3
Deformity reduction occurs through correcting the position of the hallux, which allows for an associated reduction in the intermetatarsal (IM) angle. However, the amount of IM angle reduction (and therefore bunion correction achievable) varies in the literature with some authors noting up to nine degrees of available IM correction.5 Accordingly, let us take a closer look at the unique mechanism of IM angle correction and reassess the amount achievable when performing a first MPJ arthrodesis.
While various authors have attempted to describe the biomechanical theory of IM angle correction after a first MPJ fusion, there is a lack of experimental attempts to illustrate this concept.5-8 The most likely osseous explanation behind the IM angle correction is the removal of the retrograde force by the hallux that pushes the first metatarsal medially.5 Furthermore, Mann and Katcherian also observed that an osseous fusion/stabilization eliminated the bowstringing effect of the long flexor and extensor tendons, thereby reducing their deforming forces.6 From a soft-tissue perspective, the conjoined tendon of the adductor hallucis also plays a role in IM reduction. It pulls the first metatarsophalangeal joint toward the midline, preventing the lateral deviation of the hallux.7 This is due to the adductor hallucis acting on a lever arm. When the joint is fused, the tendon pulls laterally on both the head of the metatarsal and base of proximal phalanx.7 A study by Pydah and colleagues further backed this theory that the adductor hallucis tendon helps decrease the IM angle, but they noted this is only true if the proximal first metatarsocuneiform joint is flexible.8 These literature findings demonstrate that a combination of osseous and soft tissue biomechanical forces help reduce the IM angle after a first MPJ fusion.
How Much IM Angle Correction Can One Achieve After First MPJ Fusion?
The amount of intermetatarsal angle correction after a first MPJ fusion has varied significantly from study to study. In 2006, Cronin and colleagues demonstrated a mean of 8.22 degrees of IM angle correction after a first MPJ fusion using either crossed screws or a dorsal plate and screw construct.7 Similarly, McKean and colleagues reported a mean IM angle change of 8.3 degrees in a 2016 study.9 Additionally, a group in France cited a mean reduction of 7.7 degrees of IM angle correction utilizing a compression screw and plate fixation construct.10
Studies by Dayton and Coughlin and their respective teams demonstrated approximately six degrees of IM angle correction after a first MPJ fusion.11,12 It is important to note that this mean IM correction stayed consistent in both studies despite the differences in the study design. In the study by Coughlin and colleagues, there was a greater range of preoperative IM angles (10 to 25 degrees). All the patients in the study by Dayton and coworkers had a preoperative IM angle of greater than 15 degrees.11,12 In 2014, Feilmeier and colleagues reported a mean decrease in IM angle of 5.44 degrees after a first MPJ fusion.5
However, there is a large outlier with regards to reported IM correction, noted by Sage and colleagues in Chicago. They noted a mean IM angle reduction of only 1.7 degrees after first MPJ fusion in their 1997 report.13 This is the lowest reported IM angle correction after the first MPJ arthrodesis procedure in the literature.
Conclusively, in a 2014 systematic review, Dayton and colleagues evaluated 15 studies with a total of 701 first MPJ fusions and found a mean of 4.36 degrees of IM angle correction after first MPJ fusion.14 This value may be the most accurate in that it is well approximated at the midline between the range of values described in the previous paragraphs. Overall, these reports demonstrate that one can achieve a wide variety of IM angle correction by different fixation methods.
Does IM Angle Correction After First MPJ Fusion Change Over Time?
After a first MPJ fusion, IM angle correction generally should not increase or decrease over time due to the stability of the construct. However, there are conflicting findings on this subject in the literature.
Costa and colleagues showed that after first MPJ fusion, the immediate postoperative IM angle decreased by six degrees from its preoperative value.15 However, they noted a loss of correction of 0.2 degrees at a mean final follow-up of 58 months. They attributed this to manual measurement error and the immediate post-op X-ray being non-weightbearing versus a later weightbearing post-op X-ray.15
However, other researchers have noted a potential for achieving significant IM angle correction over time. In the aforementioned work by Cronin and colleagues, the authors found that 20 patients with a mean preoperative IM angle of 16.65 degrees had a mean 6.3 degrees of IM angle correction at a six-week follow-up after first MPJ arthrodesis.7 At the final follow-up, there was a mean increase in IM correction from 6.3 to 8.22 degrees.7 This means that from the six-week post-operative period to the mean final follow up of 13.7 months, there was a mean IM angle improved with a decrease of 1.68 degrees. With four of the patients, however, Cronin and colleagues noted an additional four degrees of IM angle improvement. They suggested that adductor hallucis pull led to this change.7 This study reinforces the idea that all IM angle correction may not occur during surgery and therefore could improve over time.
Does IM Correction Increase With The Severity Of The Bunion Deformity?
There are numerous studies that demonstrate that the more severe the bunion deformity is, the greater the correction of the IM angle that one can achieve with a first MPJ fusion.
Dalat and colleagues noted a statistically significant gain in correction achieved from patients with a pre-op IM angle of less than 15 to even greater than 20 degrees.10 Cronin and team also found a correlation between a larger preoperative IM angle and a greater amount of IM angle correction achieved with the first MPJ arthrodesis.10 For patients who had a preoperative IM angle less than 15 degrees, the study authors noted a mean change in IM angle of 6.13 degrees after first MPJ fusion. However, when the preoperative IM angle was greater than 16 degrees, they observed an average IM angle change of 9.9 degrees.10
Feilmeier and coworkers also had similar findings, noting a 4.21 degree post-op change in IM angle in those with a preoperative IM angle of 11-15 degrees.5 For patients who had a pre-op IM angle ranging between 16-24 degrees, the study authors saw a mean 6.83 degree reduction in the IM angle after first MPJ fusion. The achievable amount of IM angle change was statistically significant between both groups, indicating greater achievement of correction with greater pre-op IM deformity.5
Mann and Katcherian also reported similar findings.6 In patients with a pre-op IM angle of less than 12 degrees, from 13-19 degrees, and greater than 19 degrees, the change in post-op IM angle was 2.4, 6.6 and 8.8 degrees respectively.6 In a 2010 study, Sung and team confirmed statistical significance in the differences of achieved IM angle correction between mild, moderate and severe pre-op bunion deformities.16
These studies demonstrate that surgeons can achieve greater IM angle correction with first MPJ arthrodesis in patients with more severe deformity. In all of these studies the authors demonstrated that because greater correction was achievable in severe bunions, a secondary base osteotomy may not always be necessary.
In Conclusion
Not only is the first MPJ fusion a reliable procedure for treating arthritic conditions of this joint, but surgeons may also utilize this procedure to help correct associated bunion deformities. With adequate fixation, performing the first MPJ arthrodesis can provide significant correction of the first IM angle, preventing a need for further osteotomies in the first metatarsal. First MPJ arthrodesis allows for successful surgical treatment of all stages of bunion deformities.
Dr. Hoffler is a second-year resident with the Podiatric Medicine and Surgery Residency Program at Wake Forest Baptist Medical Center in Winston-Salem, N.C.
Dr. Shaffer is a second-year resident with the Podiatric Medicine and Surgery Residency Program at Mercy Hospital and Medical Center in Chicago.
Dr. Hart is a second-year resident with the Podiatric Medicine and Surgery Residency Program at AMITA Health Saint Joseph Hospital Chicago in Chicago.
Dr. Burke is a second-year resident with the Podiatric Medicine and Surgery Residency Program at Mercy Hospital and Medical Center in Chicago.
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