The sigmoid notch is a shallow, concave depression on the inner side of the distal radius, the larger of the two forearm bones at the wrist. It cradles the head of the ulna, the smaller forearm bone, forming the distal radioulnar joint (DRUJ). This joint is what lets you turn your palm up and down, the movement called forearm rotation or pronation-supination. A separate structure with the same name exists on the mandible (the jawbone), sitting between the coronoid process and the condyle, but in clinical practice “sigmoid notch” almost always refers to the wrist unless a dentist or maxillofacial surgeon is talking.
Where It Sits and What It Looks Like
Picture the end of your radius bone at the wrist. On the thumb side, it broadens to meet the small wrist bones. On the pinky side, it scoops inward to form a shallow dish. That dish is the sigmoid notch. Its job is to provide a smooth articular surface against the rounded head of the ulna. Despite its importance, the notch is surprisingly small and shallow. One three-dimensional analysis found its maximum depth averaged only about 1.7 mm, with an articular width of roughly 8 mm from front to back and about 13 mm across the wrist axis.1PubMed Central. A three-dimensional analysis of the sigmoid notch The radius of curvature, a measure of how gently or tightly the notch curves, averaged around 15 mm in that study, though other analyses have found slightly larger values.2PubMed. A three-dimensional virtual morphometry study of the sigmoid notch of the distal radius
These numbers matter because the sigmoid notch is significantly less curved than the ulnar head it articulates against. The ulnar head has a tighter radius of curvature, which means the two surfaces are not perfectly matched. This mismatch allows a degree of translational sliding alongside the simple pivoting motion, which is part of what makes forearm rotation so smooth and adaptable. But the mismatch also means that only a small fraction of the notch is in contact with the ulna at any given moment. Pressure studies in cadaver forearms showed that at neutral rotation with a moderate load, only about 12.5% of the sigmoid notch surface was actually touching the ulna.3The Journal of Hand Surgery. Pressure distribution in the distal radioulnar joint The rest of the joint’s stability comes from ligaments, the triangular fibrocartilage complex, and the surrounding soft tissues.
Four Shape Types and Why They Vary
Not everyone’s sigmoid notch looks the same. Researchers have identified four morphological types based on cross-sectional imaging:
- C-type: a smooth, evenly curved surface, the most common variant in most studies.
- Flat-face: a relatively flat articular surface with minimal concavity.
- Ski-slope: a surface that angles steeply, typically with one rim much higher than the other.
- S-type: a wavy surface with both concave and convex portions.
How common each type is depends on the study population and the imaging method used. A cadaveric study of 40 specimens found 42% were C-shaped, 33% flat, 17% ski-slope, and 8% S-shaped, while CT scans from 100 living patients in the same study showed 48% C-shaped, 17% flat, 15% ski-slope, and 20% S-shaped.4Journal of Orthopaedics. Morphometry of sigmoid notch: A novel method of shape assessment for clinical practice An MRI-based study found the C-type even more dominant at 60%, with ski-slope at 30%, S-type at 6%, and flat at just 4%.5PubMed Central. Midsectional Magnetic Resonance Imaging Analysis of the Sigmoid Notch of the Distal Radioulnar Joint The flat-face type, interestingly, was not identified at all in one virtual morphometry study, suggesting that its prevalence may be influenced by how the notch is sliced and measured.6PubMed. A three-dimensional virtual morphometry study of the sigmoid notch of the distal radius
There also appear to be sex-based differences. A radiological evaluation found that sigmoid notch type distribution differed between men and women, although the C-type depth measurements were similar (averaging about 2.5 mm in women and 2.4 mm in men).7Cukurova Medical Journal. Anatomical and radiological evaluation of sigmoid notch morphometry Left and right wrists within the same person tend to be mirror images of each other in most dimensions, though one study noted a difference in the radius of curvature between sides.8PubMed. A three-dimensional virtual morphometry study of the sigmoid notch of the distal radius
How Shape Affects Stress and Injury Risk
The shape of your sigmoid notch is not just an anatomical curiosity. It influences how force travels through the wrist. A finite-element study found that ski-slope notches concentrated high-stress areas in the dorsal (back) region of the joint far more than C-type notches did. In the ski-slope group, about 16% of the dorsal region bore high-density stress, compared with only about 4% in the C-type group.9PubMed. Morphological characteristics of the sigmoid notch of the distal radius affect the stress distribution patterns in the distal radioulnar joint Uneven stress distribution over years could contribute to cartilage wear, and it may help explain why some people develop joint problems at the DRUJ while others with similar activity levels do not.
The MRI-based study that examined dorsal rim prominence found that C-type and S-type notches had more prominent dorsal rims (averaging around 1.5 mm), while flat and ski-slope types had flatter rims.10PubMed Central. Midsectional Magnetic Resonance Imaging Analysis of the Sigmoid Notch of the Distal Radioulnar Joint A prominent dorsal rim acts as a bony restraint against dorsal subluxation of the ulna, so people with flat or ski-slope types may rely more heavily on soft-tissue stabilizers to keep the joint in place. This is clinically relevant when surgeons are assessing DRUJ instability: a patient whose sigmoid notch offers less bony buttressing might need a different stabilization strategy than one with a deep, prominent C-type notch.
How the Joint Moves During Forearm Rotation
When you rotate your forearm, the radius swings around the ulna. The sigmoid notch slides over the ulnar head in a complex path that is not a simple pivot. In-vivo tracking studies found that the sigmoid notch slid over the ulnar head by up to about 7 mm during pronation and about 9 mm during supination, mostly in the front-to-back direction. Meanwhile, the ulnar head’s own sliding was more limited, around 5 mm in pronation and about 2 mm in supination.11PubMed. In vivo gliding and contact characteristics of the sigmoid notch and the ulna in forearm rotation Most of the movement happened between about 30 degrees of pronation and 60 degrees of supination, which corresponds to the arc of rotation you use most during everyday tasks like turning a doorknob or using a screwdriver.
The center of rotation itself shifts slightly during the movement. At full pronation, it sat about half a millimeter toward the ulnar side and slightly forward from the center of the ulnar fovea; at full supination, it moved slightly radially and further forward.12PubMed Central. Biomechanics of the Distal Radioulnar Joint During In Vivo Forearm Pronosupination This shifting center of rotation is one reason that DRUJ implant design is so challenging: a fixed metal hinge cannot replicate what nature designed as a gliding, translating, slightly migrating pivot.
Pressure distribution also shifts during rotation. In pronation, the contact pressure concentrates toward the dorsal (back) part of the sigmoid notch, while in supination it shifts to the palmar (front) part.13The Journal of Hand Surgery. Pressure distribution in the distal radioulnar joint This cycling of load-bearing zones likely helps distribute cartilage wear over time in a healthy joint, but it also means that a fracture or deformity affecting just one part of the notch can cause symptoms only in certain positions of rotation.
When Fractures Reach the Sigmoid Notch
Distal radius fractures are among the most common broken bones, and many of them extend into the sigmoid notch without anyone realizing it. In one study of 20 intra-articular distal radius fractures, standard X-rays identified sigmoid notch involvement in only 35% of cases, while CT scans found it in 65%. Of those fractures that reached the notch on CT, about 69% were displaced.14PubMed. Evaluation of the sigmoid notch with computed tomography following intra-articular distal radius fracture This is a meaningful gap. If a fracture line enters the sigmoid notch but goes undetected on a plain X-ray, the surgeon may not address the articular surface during fixation, potentially leaving behind a step-off or gap that could cause problems later.
The good news is that sigmoid notch involvement, on its own, does not appear to be a death sentence for joint function. One study following patients for over six years after surgical fixation found that fractures involving the sigmoid notch did not show a significantly higher rate of DRUJ arthritis compared with those that spared it. However, there was an important caveat: patients whose sigmoid notch had a residual coronal step-off greater than 1 mm after surgery reported significantly worse upper-extremity function.15PubMed Central. Intra-articular Fractures of the Sigmoid Notch of the Distal Radius: Analysis of Progression to Distal Radial Ulnar Joint Arthritis and Impact on Upper Extremity Function in Surgically Treated Fractures A more recent retrospective study echoed this finding: patients with articular step-off after volar locking plate fixation had higher arthritis grades on imaging, although their day-to-day functional scores were similar to those without step-off.16PubMed Central. Intra-articular sigmoid notch fractures of the distal radius and their association with arthritis progression and outcomes after volar locking plate fixation: A retrospective study
The practical takeaway is that when a distal radius fracture is complex enough to suspect articular involvement, a CT scan is worth getting. And during surgical repair, surgeons should aim to reduce any sigmoid notch step-off to below a millimeter. The joint can tolerate a surprising amount of irregularity without causing major functional complaints, but leaving a visible ledge on the articular surface invites long-term cartilage wear.
Surgical Options for the Sigmoid Notch
When the sigmoid notch is damaged or deformed beyond what simple fracture fixation can address, surgeons have several options, depending on severity.
For malunions where the notch healed in a wrong orientation, corrective osteotomy can recontour the articular surface. One described technique involves cutting and rotating the notch back into proper alignment with the ulnar head, restoring joint congruency and improving supination.17PubMed. Corrective osteotomy for intra-articular malunion of the sigmoid notch of the distal part of the radius: a case report A related approach involves a coronal-plane osteotomy of the sigmoid notch to correct reverse obliquity, where the notch tilts the wrong way relative to the ulnar head. This is combined with ulnar shortening in cases where the ulna also sits too long relative to the radius.18PubMed Central. Osteotomy for sigmoid notch obliquity and ulnar positive variance
For more advanced DRUJ arthritis, the Sauvé-Kapandji procedure fuses the distal radioulnar joint itself while creating a gap (pseudoarthrosis) in the ulna just above the fusion site. Forearm rotation is maintained through this gap. The approach preserves the ulnar head’s support of the wrist and maintains the normal appearance of the ulnar prominence, advantages over the alternative Darrach procedure, which simply removes the ulnar head. One series of 27 Sauvé-Kapandji procedures reported that about 59% of patients had mild or no pain at one year, the pronation-supination arc improved from an average of about 97 degrees preoperatively to about 136 degrees postoperatively, and grip strength reached roughly half the strength of the opposite wrist.19PubMed. Sauvé-Kapandji procedure in distal radioulnar joint disorders Complications can include nonunion at the fusion site or bony re-bridging at the pseudoarthrosis, which would block rotation again.20PubMed Central. The sauvé-kapandji procedure
Total Joint Replacement at the DRUJ
When salvage procedures fail or the joint is too destroyed for reconstruction, total DRUJ arthroplasty is an emerging option. These implants replace both the ulnar head and resurface the sigmoid notch, aiming to restore a functional articulation. One early report on the STABILITY Sigmoid Notch Total DRUJ System followed four patients for an average of about four years. Pain scores dropped substantially (from 8 out of 10 preoperatively to 2.5 postoperatively), and grip strength nearly doubled from about 14.5 kg to 25.5 kg. Pronation averaged 80 degrees and supination 64 degrees, with no cases of implant subluxation or dislocation.21PubMed Central. Preliminary experience with a new total distal radioulnar joint replacement
A separate series of nine patients who underwent ulnar head replacement with sigmoid notch resurfacing showed Mayo Wrist Scores improving from 37 to 73 (out of 100), pain dropping from 7 to 1, and grip improving from 20 kg to 30 kg. Six of nine patients achieved a good or excellent outcome, though two required revision surgery to improve wrist motion.22PubMed. Ulnar head replacement and sigmoid notch resurfacing arthroplasty with minimum 12-month follow-up These are small case series, and total DRUJ replacement remains a niche procedure performed at specialized centers. The biggest design challenge, as mentioned earlier, is replicating the joint’s complex, shifting kinematics with a mechanical device. But for patients who have exhausted other options, it represents a meaningful step up from living with a destroyed joint.
Better Imaging and 3D Planning
One of the recurring themes in sigmoid notch research is that two-dimensional imaging often falls short. Standard X-rays miss sigmoid notch fractures, underestimate displacement, and cannot classify notch morphology reliably. Three-dimensional automated measurement tools are being developed to address this. One system used CT-derived 3D models to measure notch angle and ulnar variance, achieving a high correlation with conventional radiograph measurements (0.77 for the notch angle, 0.85 for ulnar variance) while adding the ability to quantify notch morphology independent of forearm position or radius orientation.23PubMed. Three-Dimensional Automated Assessment of the Distal Radioulnar Joint Morphology According to Sigmoid Notch Surface Orientation This kind of tool could eventually make it routine to classify a patient’s sigmoid notch type before surgery, allowing the surgeon to tailor the fixation strategy, implant selection, or osteotomy angle to the individual anatomy.
The same 3D analysis revealed that notches with a negative angle (those tilting one way) had a larger radius of curvature (about 19 mm) than those tilting the opposite way (about 17 mm), reinforcing the idea that notch geometry varies more than most clinicians appreciate. As 3D printing and patient-specific surgical guides become more accessible, having an accurate 3D map of the sigmoid notch could become standard preoperative preparation for complex DRUJ procedures.
The Other Sigmoid Notch, on the Jawbone
The mandible has its own sigmoid notch, also called the mandibular notch, sitting between the coronoid process in front and the condylar process in back. It is a concavity at the top of the ramus (the vertical part of the jawbone), and the masseteric nerve and vessels pass through it. While less studied from a surgical standpoint than its wrist counterpart, the mandibular sigmoid notch has clinical relevance in oral and maxillofacial surgery, particularly in forensic identification.
A cone-beam CT study of 100 images (200 sides) found that the mandibular sigmoid notch was round in shape in about 40.5% of cases, making it the most common variant in that sample.24Cureus. Distinctive Anatomical Patterns of the Mandibular Coronoid Process, Condyle, and Sigmoid Notch: Cone Beam Computed Tomography (CBCT) Imaging for Advanced Personal Identification In children, the shape changes with growth. During the primary and mixed dentition stages (roughly ages 4 to 12), the mandibular sigmoid notch tends to be wide, gradually transitioning to a rounder form as permanent teeth come in and the jaw matures.25Contemporary Pediatric Dentistry. Morphological variations of mandibular sigmoid notch in children aged 4-14 years These developmental changes are of interest to pediatric dentists monitoring jaw growth and to forensic specialists who use mandibular landmarks for age estimation and personal identification.
Evolutionary Clues From the Forearm
The DRUJ and its sigmoid notch are not universal among primates. The ability to rotate the forearm extensively through pronation and supination is linked to the evolution of the elbow and wrist complex in hominoids (great apes and humans). Fossil evidence from the Miocene great ape Hispanopithecus laietanus shows an elbow complex capable of a broad range of pronation and supination, consistent with suspensory behaviors like swinging from branches. Yet the same fossil retained features associated with quadrupedal walking, suggesting a transitional locomotor stage in which the forearm’s rotational machinery served double duty.26PLoS ONE. A Partial Skeleton of the Fossil Great Ape Hispanopithecus laietanus from Can Feu and the Mosaic Evolution of Crown-Hominoid Positional Behaviors The sigmoid notch as we know it in humans, a refined concavity tuned for smooth and extensive rotation, is the result of millions of years of selection for both arboreal agility and, later, the tool-using dexterity that defines our lineage.