Risk Factors for First Tarsometatarsal Joint Slippage after Third-Generation Minimally Invasive Bunionectomy: A Weight-Bearing CT Scan Assessment

Category: Bunion; Midfoot/Forefoot Introduction/Purpose: Third-generation minimally invasive (MIS) bunionectomy requires a lateral shift of the metatarsal head, pushing the shaft into adduction and levering off the 1st-tarsometatarsal (TMT) joint. A previous study on plain radiographs reported postoperative 1st-TMT joint slippage in the transverse plane, with a medial subluxation of the 1st metatarsal base. However, risk factors and clinical implications have yet to be determined. This study sought to assess the prevalence of postoperative 1st-TMT joint slippage in the transverse plane and to identify associated risk factors using weight-bearing CT (WBCT) scans. In addition, the effect of 1st-TMT slippage on postoperative midshaft foot width was investigated. We hypothesized that postoperative 1st-TMT slippage in the transverse plane would be associated with preoperative 1st-TMT instability and increased postoperative midshaft foot width. Methods: Sixty-three consecutive patients who underwent MIS bunionectomy with available pre- and postoperative WBCT-scans were included. Radiographic measurements included hallux valgus angle(HVA), intermetatarsal angle(IMA), and midshaft foot width, measured from the 1st metatarsal shaft to the 5th metatarsal head on AP-foot radiographs. Tri-planar rotation angle(TAP), and 1st-TMT plantar-gap and medial-middle(MM) intercuneiform distances(mm) were obtained from WBCTs. Two raters measured pre- and postoperative 1st-TMT distances between the most lateral aspect of the medial cuneiform and 1st metatarsal base at the dorsal, central, and plantar thirds on axial cuts. The average corresponds to the overall 1st-TMT lateral distance, a measure of 1st-TMT slippage. Rater agreement was assessed. Paired t-tests were used to compare pre- and postoperative values. Logistic regression models were used to investigate the association of postoperative 1st-TMT lateral distance with potential risk factors (preoperative 1st-TMT lateral distance, 1st-TMT plantar-gap distance, IMA, TAP, and MM intercuneiform distance) and postoperative midshaft foot width. Results: The interobserver reliability of the 1st-TMT lateral distance was excellent (r=0.97). Mean (SD) 1st-TMT lateral distance increased from 0.26 mm(0.4) to 1.33 mm(0.9) after surgery(P< 0.05). Postoperative 1st-TMT lateral distance was > 2 mm in 15 patients(24%), and > 1 mm in 33(52%). Mean(SD) dorsal 1st-TMT lateral distance (2.7mm[1.3]) was significantly higher than central (0.85mm[1.1]) and plantar (0.55mm[0.99]) 1st-TMT lateral distances. Mean(SD) midshaft foot width increased from 80.04mm(6.7) to 84.25mm(6.1) postoperatively (P< 0.05). Regression analysis showed that pre- and postoperative 1st-TMT lateral distances were correlated (Coef=0.64,P=0.02), and identified a 1 mm increase in postoperative 1st-TMT lateral distance with each 0.63 mm increase in preoperative 1st-TMT plantar-gap distance (Coef=0.63,P=0.008). A significant positive correlation was also observed between postoperative 1st-TMT lateral distance and postoperative midshaft foot width (Coef=1.43,P< 0.05). Conclusion: Our study suggests a possible association between postoperative 1st-TMT joint slippage and preoperative 1st-TMT multiaxial instability following MIS bunionectomy. This raises an important question as to whether alternative techniques, such as modified Lapidus, should be preferred over MIS bunionectomy in patients with 1st-TMT instability. Notably, medial slippage was significantly higher at the dorsal aspect of the 1st-TMT joint, suggesting a component of rotational instability. It appears that 1st-TMT slippage is associated with a wider midshaft foot width postoperatively, and patients should be warned about it. Future studies assessing mid-to-long-term effects of 1st-TMT slippage, like recurrence or degenerative changes, are warranted.