STT Arthritis Thông tin In-depth

This section goes further than you need for your own treatment decisions. STT arthritis is worth the extra reading for two reasons: the X-ray is a poor guide to whether this joint is the one that hurts, and every operation for it trades pain relief against a change in the way the small bones of the wrist sit.

How common it is, and why the X-ray misleads

Wear in the STT joint is a frequent X-ray finding. Among 700 consecutive hand-clinic patients X-rayed for any complaint, 111 (16%) showed STT arthritis; it was more likely with age, with a gap between the scaphoid and lunate, and with arthritis of the thumb base joint [1]. In a random sample of Japanese residents aged 50 to 89, 5.3% of wrists showed it, again linked to thumb-base arthritis, female sex and age [2]. Most of these people have no symptoms from the STT joint.

The X-ray is unreliable in both directions. When surgeons looked directly at the joint during thumb-base surgery, 62% of thumbs had arthritis between the scaphoid and trapezoid, but the X-ray had detected only 44% of them [3]. A larger series of 257 patients found end-stage wear in 31% at operation, with an X-ray sensitivity of 47% and specificity of 94% [4]. Moderate STT wear left alone at the time of thumb-base surgery did not progress or cause symptoms over a mean of 8 years in one long-term series [5]. This is why the decision rests on where the tenderness is, and often on a guided injection, rather than on the X-ray alone.

The tilt of the wrist bones

STT arthritis is linked to a particular tilt of the wrist bones, called dorsal intercalated segment instability (DISI): the lunate tips backwards while the scaphoid sits more upright. In 135 wrists with STT arthritis, DISI was present in 5% of moderate cases and 60% of severe ones; unlike the tilt that follows a torn scapholunate ligament, the gap between the scaphoid and lunate stayed normal and the main wrist joint did not wear [6]. Some authors now describe a pattern of collapse driven by STT arthritis (SOAC wrist) as distinct from the better-known SLAC wrist [7].

It matters for surgery because an operation that removes bone can increase the tilt. A wrist that is already unstable in the middle row of bones can collapse after a simple excision, as in a reported case that needed a later fusion [8]. Surgeons therefore look for the tilt and for midcarpal instability before choosing an operation.

Distal scaphoid excision: relief, at the cost of a tilt

Removing the end of the scaphoid relieves pain in most series, and the wrist bones usually tilt afterwards. In one of the first series, 12 of 21 wrists developed DISI, none had further joint deterioration, and satisfaction was the same as in wrists that did not tilt [9]. A later series found DISI in 19 of 21 wrists (95%), with the angle of the lunate rising by a mean of 13°, and no relation between the tilt and the functional scores at a mean of 3 years [10].

How much bone is removed matters. After arthroscopic excision in 17 wrists, pain scores fell from 6.1 to 1.7 out of 10 and pinch strength rose from 2.5 kg to 4.4 kg; the tilt was more likely when more than 3 mm of bone was taken [11]. In cadaver wrists, removing a quarter of the scaphoid barely changed how the bones moved, half caused a clear tilt, and three-quarters made the wrist unstable [12]. An anatomical study found that the deep band of the scaphotrapezial ligament attaches about 3.3 mm from the end of the scaphoid, which gives an anatomical reason to keep the resection to about 3 mm [13]. A cadaver study of the same operation measured the capitate rotating back by about 14° and the lunate by 8° [14]. Rarely, a sharp edge left on the scaphoid has worn through a flexor tendon [15].

When the thumb base is worn as well

When both the thumb base and the STT joint are worn, the usual operation removes the trapezium and the worn end of the trapezoid. In 37 thumbs, adding a partial trapezoid excision caused no extra problems compared with trapezium removal alone [3]. How much trapezoid to remove is the debated point. In 79 hands followed for a median of 92 months, removing part or all of the trapezoid made a backward tilt of the lunate of 15° or more more likely (29% after partial and 26% after complete removal), and wrists with that tilt scored worse; symptomatic sinking of the index metacarpal occurred in 18% after complete and 10% after partial removal, and the authors advised against complete removal [16]. A cadaver study found that 4 to 6 mm of trapezoid has to be removed before the pressure between the scaphoid and trapezoid falls in bending and side-to-side movements [17]. In a series of 150 thumbs followed for a mean of 13.5 years, trapeziectomy with partial trapezoidectomy and a tendon reconstruction gave a mean DASH disability score of 8.7 and no revisions [18].

Pyrocarbon implants

A pyrocarbon spacer aims to relieve pain while keeping the height of the scaphoid, and so to limit the tilt. With the Pyrocardan disc, 20 patients (22 implants) followed for a mean of 3.8 years had less pain, better scores and stronger pinch, with some loss of wrist extension and no dislocations; the capitolunate angle improved rather than worsened [19]. A series followed for a mean of 5 years reported 95% of implants free of reoperation, one painless dislocation, calcium around the joint in 4 wrists, and a tilt that persisted in 3 and appeared in 1 [20]. Compared with STT fusion in a non-randomised study, results were similar except for better wrist extension with the implant (58° against 38°); 22% of implant wrists developed a tilt, and 9% of fusions failed to unite [21]. Most of these series come from a small number of French centres.

Results with other designs have been less consistent. With a different pyrocarbon implant shaped to replace the end of the scaphoid (the STPI), 7 of 29 implants were revised, mainly for pain, and the number of wrists with a tilt doubled from 6 to 12 over a median of 68 months [22]. A case report describes a Pyrocardan that broke twice in a tennis player, which is why heavy loading of the wrist in extension is discussed before surgery [23].

STT fusion, for comparison

Fusion joins the three bones into one. Across 1,429 operations in a systematic review, the fusion failed to unite in 6.3%, 4.2% were converted to a full wrist fusion, and grip strength averaged 76% of the other hand [24]. In 58 wrists followed for 10 years, 10% needed a further operation and 7% had a painful non-union [25]. Fusion typically keeps about three-quarters of wrist movement, and pain from the radial styloid catching on the fused scaphoid is common enough that some surgeons trim the styloid at the same time [26].

What is not known

There are no randomised trials comparing these operations. A survey of European hand surgeons found a wide spread of preferred operations for the same problem [27], and recent reviews conclude that no option has clear evidence of superiority; the choice depends on whether the STT joint is worn alone, whether the thumb base is worn too, and whether the wrist bones are already tilted [28] [29]. Most series follow fewer than 30 patients for less than 10 years, so the long-term effect of the tilt after excision, and the long-term survival of pyrocarbon implants, are still open questions.