Since apparent structural abnormalities, such as a malunited distal radius fracture, radial head resection, and Madelung deformity, are seen in secondary UIS, choosing surgical treatment is not difficult [1]. However, it is almost impossible to recognize patients whose conditions would be improved by conservative treatment or will inevitably require surgical treatment for idiopathic UIS. The consensus to decide on surgical treatment for idiopathic UIS is when the pain or disability is not improved even after appropriate conservative treatment[6–8]. In this study, the authors also managed all UIS cases conservatively for at least 6 months. After that, surgical treatments were chosen for patients without pain or disability improvement. If UIS may not respond to conservative treatment, it will be wastes of time and cost. Identifying factors associated with pain intensity or functional limitation after conservative treatment can help predict inevitable surgeries and decide immediately on surgical or non-surgical treatment of UIS at the first visit. This is the first study to predict the conservative treatment failure and inevitable surgery in UIS with clinical and radiologic findings.
Based on our findings, daily work-related exposure was the only significant predictor of pain intensity (VAS score) and subjective functional limitation (DASH score) after the conservative treatment of UIS (Table 2,3). This means that high work exposure results in the failure of conservative treatment in UIS, and surgical treatment would be inevitable in these patients. On the contrary, a longer ulnar variance did not affect the results of the conservative treatment. Since it has been known that UISs are frequently associated with longer ulnar variance [1, 18], the longer ulnar variance has been considered to induce more significant symptoms in UIS. However, the longer ulnar variance did not always cause more significant pain and subjective functional limitation in this study, even though the longer ulnar variance has a greater chance of impinging with ulnar carpus. This seems to be because the impinging occurs only during wrist movement, and an immobile wrist can not induce the impinging [18]. Positive ulnar variance with lower work-related exposure can be asymptomatic, whereas neutral or even negative ulnar variance with higher work-related exposure can be symptomatic[1]. Repetitive wrist movement and load is the key to inducing symptoms of UIS.
The degenerative complex TFCC tear was a common predictor of objective functional limitation (both limited active wrist motion and diminished grip strength) in the present study (Table 4,5). The UIS with degenerative complex TFCC tears tended not to improve with conservative treatment. Degeneration commonly progresses slowly over a long time. Therefore, the ulnocarpal degeneration includes central wear or perforation of the TFCC with a smooth margin and focal chondromalacia of bony cartilage in UIS (Fig. 2) [2]. However, the degenerative complex TFCC tear in this study had a complex tear configuration accompanied by a severely degenerated tear margin and fragmented detachment of the bony cartilage that differed from usual TFCC and cartilage lesions in UIS (Fig. 3). Moreover, none of the patients included had a history of trauma on their wrist, and a degenerative complex tear could be found at any stage of ulnocarpal degeneration in this study. Hence, it was thought that the degenerative complex TFCC tear was caused by neither a usual degeneration process nor trauma of the ulnocarpal joint. Similar to a degenerative complex meniscus tear of the athlete's knee [19, 20], the degenerative complex TFCC tear seems to have occurred because of excessive load concentration in the ulnocarpal joint during a short time. This was also associated with excessive wrist use over a short time. The excessive load transfer across the ulnocarpal joint was increased during repetitive wrist ulnar deviation, forearm pronation, and a powered hand grip [21, 22]. In this situation, the TFCC is torn rather than worn or perforated. In the present study, 42 degenerative complex TFCC tears were found on arthroscopy of 177 wrists, and they consisted of three types of tear components (horizontal, radial, and flap tear) (Fig. 4). Complex TFCC tears can cause mechanical irritation and consequential pain in the ulnocarpal joint during wrist motion or grip[19, 20, 23, 24]. However, mechanical irritation is not induced if patients do not move their wrists. This is the main reason for the objective functional limitation [19, 20, 23, 24]. In a previous study, a longer ulnar variance did not affect the clinical outcome of TFCC repair, and degenerative TFCC tear is associated with unfavorable clinical outcomes of TFCC repair [25, 26]. This also supports our findings that it is not a longer ulnar variance but a degenerative complex TFCC tear associated with UIS symptoms. The mechanical irritation of the degenerative complex TFCC tear will not improve with conservative treatment, and it results in inevitable surgery such as arthroscopic debridement or repair [27, 28].
The longer ulnar variance was only associated with limited wrist motion. The ulnar variance is increased during wrist ulnar deviation and forearm rotation[15, 21, 22]. This induces impingement of the ulnocarpal joint, which is the reason for this limitation. The Palmar class 2D and 2E degeneration also reduced the grip strength in this study. In the Palmar classification system, the presence of the lunotriquetral ligament tear differs from 2B or 2C class degeneration [2]. The lunotriquetral ligament tear can elicit dynamic instability of the lunotriquetral joint [29]. The lunotriquetral instability can cause pain during hand grip, reducing its strength [29].
This study has one limitation. First, the questionnaire for assessing the amount of work-related exposures in this study was modified from the Nordstrom questionnaire and not specific for patients with UIS. The original Nordstrom questionnaire is the patient's self-reported sum of the daily occurrences of seven physical exposures for carpal tunnel syndrome [11–13]. Since six of the seven items were related to UIS and we excluded irrelevant item (time of use of handheld vibrating power tool) from the Nordstrom questionnaire to make it more suitable for UIS in this study [11–13], it was considered enough to quantify the work-related exposure in this study.