Scapholunate ligament

Function

Where it sits

The scapholunate ligament connects the scaphoid and the lunate¹. A ligament is a tough band of tissue that holds bones together at a joint. The scaphoid and lunate are carpal bones, the small bones of the wrist². Counting from the thumb side, they are the first two bones in the row next to the forearm². Both meet the end of the radius, the forearm bone on the thumb side, directly².

Two labelled line drawings of the bones of a hand and wrist, one seen from the front and one from the back. At the base of each hand, above the ends of the radius and ulna, sit the eight carpal bones in two rows. The row next to the forearm is labelled scaphoid, lunate, triquetrum and pisiform; the row next to the palm is labelled trapezium, trapezoid, capitate and hamate. Beyond them are the five numbered metacarpals of the palm and the phalanges of the thumb and fingers. A small inset marks the carpals, metacarpals and phalanges on a whole hand. — Bones of the wrist and hand, front and backBetts JG, Young KA, Wise JA, et al. Anatomy and Physiology, Figure 8.7. OpenStax, 2013. Access for free at openstax.org. (source, CC BY 4.0). Resized from 1954 to 1400 pixels wide; nothing else changed.

The drawing shows the bones without their ligaments; this ligament joins the two labelled scaphoid and lunate¹. Intrinsic wrist ligaments are those that run from one carpal bone to another⁷. A review of wrist anatomy calls the scapholunate and lunotriquetral ligaments the most important of them⁷.

Three parts, built differently

A review describes the ligament as C-shaped, with three structurally distinct parts: volar (palm side), membranous (thin and sheet-like) and dorsal (back)⁴. A study of cadaver wrists from adults, 21 fresh and 16 fixed, could consistently divide it into dorsal, proximal and palmar regions¹. The two sources name the parts differently^1,4. Proximal means nearer the centre of the body, and palmar means on the palm side. That study found each region built differently:

The dorsal region was thick, with short fibres of collagen (a strong, rope-like protein) running straight across from bone to bone¹.
The palmar region was thin, with collagen bundles running at a slant¹.
The proximal region was mainly fibrocartilage, a tough, fibrous kind of cartilage¹.

What it holds steady

In eight cadaver arms moved in a wrist joint simulator, the ligament appeared to be the primary stabiliser between the scaphoid and lunate⁸. Cutting it changed how the two bones moved during wrist flexion and extension, the bending of the wrist forward and back⁸. It did not change their motion during side-to-side tilting⁸.

In 24 cadaver wrists, the dorsal region gave the greatest restraint to the two bones sliding against each other³. In sixteen fresh cadaver wrists, cutting the dorsal portion accounted for 37% to 72% of the extra scaphoid flexion seen after cutting the whole ligament⁹. Cutting the volar portion accounted for 6% to 14%, depending on the motion tested⁹.

Other ligaments back it up. In five fresh cadavers, completely cutting the scapholunate ligament did not by itself produce a static collapse of the lunate¹⁰. After the dorsal intercarpal ligament was also detached from the lunate, both bones moved and the gap widened¹⁰. That ligament crosses the back of the wrist and joins several wrist bones.

How strong it is: studies disagree

Three cadaver studies loaded parts of the ligament until they gave way, and their figures differ^3,11,12. Forces are in newtons, a unit of force.

In preparations from 24 adult cadaver wrists, the dorsal region had the greatest yield strength, about 260 newtons³. The palmar region followed at about 118 newtons and the proximal region at about 63³.
In ligaments from 10 fresh-frozen cadavers, the dorsal 10 mm pulled to failure had a peak load of 270 ± 91 newtons¹¹.
In one study of ligaments from fresh frozen cadaver wrists, the dorsal and palmar portions failed at mean loads of 83 and 86 newtons¹². There was no statistical difference between them, and the intermediate portion failed at 36 newtons¹².

Where its nerve endings lie: studies disagree

Mechanoreceptors are nerve endings that sense pressure, stretch or movement. Two small cadaver studies place them differently in this ligament^13,14:

The authors of a study of six fresh cadaver ligaments concluded that the palmar subregion holds the most nerve structures and mechanoreceptors¹³. They described the dorsal subregion as densely packed collagen with limited nerve supply¹³.
In ligaments from 5 fresh-frozen cadaver specimens, nerve endings and mechanoreceptors were pronounced in the dorsal wrist ligaments, the dorsal scapholunate ligament among them¹⁴. They were limited or occasional in most volar wrist ligaments¹⁴.

In nine volunteers with a mean age of 26, the dorsal part was stimulated electrically through a fine-wire electrode¹⁵. Forearm wrist muscles were excited within 20 milliseconds¹⁵. Co-contraction, opposing muscles working at the same time, peaked around 150 milliseconds¹⁵.

Breakdown

How it tears

A review describes the injury as common in young people⁴. It is often the result of a fall forwards from a height, a sports injury or a motorcycle accident⁴. Another review says the injuries that separate the two bones are thought to follow one kind of fall most often¹⁶. The wrist lands bent back and tilted towards the little-finger side¹⁶.

An experiment loaded 32 cadaver wrists to failure¹⁷. Dislocations around the lunate came from wrist extension, ulnar deviation and intercarpal supination¹⁷. Ulnar deviation tilts the hand toward the little finger, and intercarpal supination is a twist between the wrist bones. A dislocation is when a bone is forced fully out of its normal place at a joint. The first and least unstable stage of that damage was a gap between the scaphoid and lunate¹⁷.

How often it is injured

One review calls this the most commonly injured ligament between the wrist bones⁴. It states that about 5% of wrist sprains come with a scapholunate tear⁴. A sprain is a stretched or torn ligament. Among 144 patients with wrist pain after an injury but normal standard X-rays, arthroscopy showed scapholunate ligament lesions in 14¹⁸. Arthroscopy is keyhole surgery inside a joint, using a small camera.

Tears alongside a broken wrist

Arthroscopy studies have counted tears of this ligament alongside a distal radius fracture^19,20,21. The distal radius is the end of the radius farther from the centre of the body.

Who was studied	Scapholunate ligament finding
60 patients with a displaced fracture into the joint surface	19 had a tear¹⁹
50 initially displaced fractures in young adults	27 (54%) had a partial or total tear²⁰
A systematic review of 20 articles and 1,346 patients with an acute fracture	The overall rate of injury was 35.3%²¹

A systematic review is a study that searches for all the research on a question in a set, checkable way and sums it up. A systematic review of eleven studies looked at the ligament alongside an acute scaphoid fracture²². Reported rates of injury ran from 0% to 71%, and were higher with arthroscopy than with X-ray or MRI²². Complete tears were rare, and the overall quality of the studies was low to moderate²². The scaphoid fracture page covers that break.

Defects in wrists without symptoms, and in donated wrists

One study compared painful and painless wrists in the same people, using arthrograms (joint X-rays taken with dye)²³. Its 30 patients had wrist trauma, pain in one wrist, normal X-rays and a scapholunate defect in at least one wrist²³. The study took no other patients²³. Defects were found in 28 painful and 20 painless wrists²³. The defect reached the dorsal part in 64% of painful wrists and 25% of painless ones²³.

In 393 dissected wrists, whose symptoms in life were not recorded, the ligament was torn in 28%²⁴. In 62 cadaver wrists examined for changes in elderly wrists without symptoms, a scapholunate lesion was found in 18⁵. Most of those defects were incomplete and lay in the central portion⁵. The authors concluded that such defects arise as a process of ageing⁵.

From a tear to a collapsed wrist

A review describes four groups of scapholunate injury, by what X-rays show, and attributes them to Watson and colleagues¹⁶:

Predynamic instability. X-rays show no malalignment, which means bones sitting out of their usual line¹⁶.
Dynamic instability. The gap between the two bones may widen on a clenched-fist X-ray, or on live imaging while the wrist is moved or loaded¹⁶.
Static scapholunate dissociation. The ligament is completely torn and the bones are malaligned on plain X-rays¹⁶.
Scapholunate advanced collapse (SLAC). Its own stages follow how far arthritis has spread through the wrist¹⁶.

Another review says static instability is often called dorsal intercalated segment instability, or DISI⁴. That review states that a long-standing injury with separation of the two bones always ends in DISI and a SLAC wrist⁴. It also says the average time from injury to a SLAC wrist is unknown and variable⁴. From experience, it says, the collapse most often turns up three to 15 years after the injury⁴.

Recovery

What reviews say about healing

Three reviews comment on how a torn ligament fares^4,6,25:

One describes it as a true intra-articular ligament, one inside the joint, bathed in joint fluid⁶. It says this contributes to its poor healing potential⁶.
A second states that the intrinsic wrist ligaments tend to degenerate rapidly after injury, in as little as two to six weeks⁴. It adds that the ideal time for sewing the ligament back together has not been defined⁴.
A systematic review states that once more than 6 weeks have passed the instability counts as chronic, because the ligament can no longer heal²⁵.

Tears left alone

Two small series have followed tears that were left alone^26,27. A 2013 review states that the natural history of these injuries is essentially unknown⁶.

Eleven patients, isolated injury. Each had an injury seen at arthroscopy, no X-ray signs of DISI or gapping, and no further procedure²⁶. At an average of 7 years, all 11 still had pain and functional limitation²⁶. Not one had progressed rapidly to SLAC-type degenerative change²⁶.
Fifty-one patients under 60 with a displaced distal radius fracture. Thirty-two had a tear that was left alone: 10 complete and 22 partial²⁷. Among the 38 patients who came back 13 to 15 years later, not one had developed a static dissociation or a SLAC wrist²⁷. The study graded tears on Lindau’s scale, and no tear reached grade 4²⁷. The authors note that grade 4 tears may fare differently²⁷.

After surgery, in outline

A systematic review covered return to sport or work after surgery for isolated injury, in adults without osteoarthritis²⁸. Osteoarthritis is a long-term joint disease in which the cartilage lining the joint wears away. Across fourteen papers and six operations, return rates were above 80% for each technique²⁸. The review concluded that the best operation remains undetermined, because the published studies are varied and small²⁸. The wrist sprain page gives an outline of the reconstruction reviews.

Connections

The ligament joins the scaphoid to the lunate, inside the wrist joint^1,6. Both bones meet the end of the radius, described on the distal radius page². The scaphoid and lunate sit in the row of carpal bones next to the forearm². A review of wrist biomechanics states that no muscle acting on the wrist attaches to that row⁷. The triangular fibrocartilage complex sits between the lunate, the triquetrum and the head of the ulna⁷.

Injury to the ligament is described on the wrist sprain page, and it can accompany a distal radius fracture or a scaphoid fracture^4,21,22.

Related exercise pages: push-up. One study loaded eight fresh cadaver wrists in push-up positions²⁹. Cutting the ligament moved the centre of pressure between radius and scaphoid an average of 1.4 mm towards the thumb side²⁹.