The M-Shaped Deformity: A Novel Classification of Posterior Trunk Contour and Its Surgical Implications in Body Contouring

Introduction: Posterior trunk contour deformities at the lower back-to-gluteal transition are under-described in the body contouring literature, despite their impact on overall silhouette and procedure planning. A reproducible pattern characterized by a central midline depression at the mid buttock, flanked on each side by superior and inferior soft tissue prominences along the lateral buttock, has been observed in clinical practice and is termed the M-shaped deformity. This study defines the M-shaped deformity, proposes a classification system, and evaluates its implications for surgical planning and procedure selection in body contouring surgery. Materials and Methods: A retrospective descriptive case series was conducted of 6 female patients who underwent body contouring between 2024 and 2026 and demonstrated a characteristic posterior trunk deformity. Patients were classified into 3 deformity types based on central indentation depth, degree of skin redundancy, position of redundant tissue relative to the gluteal crease, and superior posterior trunk skin mobility on pinch testing. Preoperative and postoperative photographs were used for qualitative assessment of contour. Surgical management included circumferential liposuction (Lipo 360), standard anterior abdominoplasty or circumferential abdominoplasty, and autologous fat transfer to the hips and buttocks. Results: Two patients demonstrated Type I deformity, 2 patients had Type II deformity, and 2 patients had Type III deformity. One Type I patient was treated with Lipo 360, fat transfer, and standard anterior abdominoplasty, while the remaining 5 patients underwent circumferential abdominoplasty with Lipo 360 and fat transfer. Manual elevation of the superior posterior trunk skin improved the central depression and gluteal cellulite in all cases, supporting skin envelope descent and loss of tissue tension as the primary mechanism. Circumferential abdominoplasty produced the most consistent correction, particularly in Type II and Type III deformities, restoring a smooth S-shaped contour. No complications were observed in this series. Discussion: The M-shaped deformity is a distinct posterior trunk contour abnormality that can be reproducibly classified using a 3-tier system incorporating a simple pinch test. This classification provides a practical framework for determining when circumferential abdominoplasty, rather than standard anterior abdominoplasty with liposuction and fat transfer alone, is required to restore a smooth lower back-to-gluteal transition. Conclusion: The M-shaped deformity is a recognizable posterior trunk contour abnormality that can be classified using simple clinical features and pinch testing. Accurate identification may improve procedure selection, with circumferential abdominoplasty providing the most reliable correction in patients with greater skin descent and redundancy.