Ulnar Nerve In Situ Decompression versus Transposition for Idiopathic Cubital Tunnel Syndrome: An Updated Meta-Analysis

Round table discussion: the management of idiopathic cubital tunnel syndrome

Idiopathic cubital tunnel syndrome is the second most common neuropathy in the upper limb. Best evidence regarding the surgical management of this condition has evolved from anterior or submuscular transposition as the former reference standard, to in situ simple release. Differences of opinion remain regarding the timing of surgery, type of surgery and adjunctive surgery. Four surgeons with Level 5 expertise were asked to answer specific questions regarding this condition.

The Unpredictable Ulnar Nerve-Ulnar Nerve Entrapment from Anatomical, Pathophysiological, and Biopsychosocial Aspects

Peripheral nerves consist of delicate structures, including a rich microvascular system, that protect and nourish axons and associated Schwann cells. Nerves are sensitive to internal and external trauma, such as compression and stretching. Ulnar nerve entrapment, the second most prevalent nerve entrapment disorder after carpal tunnel syndrome, appears frequently at the elbow. Although often idiopathic, known risk factors, including obesity, smoking, diabetes, and vibration exposure, occur. It exists in all adult ages (mean age 40-50 years), but seldom affects individuals in their adolescence or younger. The patient population is heterogeneous with great co-morbidity, including other nerve entrapment disorders. Typical early symptoms are paresthesia and numbness in the ulnar fingers, followed by decreased sensory function and muscle weakness. Pre- and postoperative neuropathic pain is relatively common, independent of other symptom severity, with a risk for serious consequences. A multimodal treatment strategy is necessary. Mild to moderate symptoms are usually treated conservatively, while surgery is an option when conservative treatment fails or in severe cases. The decision to perform surgery might be difficult, and the outcome is unpredictable with the risk of complications. There is no consensus on the choice of surgical method, but simple decompression is relatively effective with a lower complication rate than transposition.

Comparison of in-situ release and submuscular anterior transposition of ulnar nerve for refractory cubital tunnel syndrome, previously treated with subfascial anterior transfer-A retrospective study of 24 cases

BACKGROUND

Although cubital tunnel syndrome is the second most common type of compressive neuropathy in the upper extremities, the indication and optimal surgical method for recurrent or refractory cubital tunnel syndrome remains controversial. This study evaluates the functional outcomes of revision surgery for cubital tunnel syndrome.

MATERIAL & METHODS

This study was a retrospective observational multicenter single-institution study including 660 patients who underwent surgery for cubital tunnel syndrome from 2010 to 2019. Among the 660 patients, 42(6.4%) received revision surgery due to remaining or recurrent symptoms confirmed with electromyography(EMG). After excluding those with concurrent elbow fracture, dislocation, osteoarthritis and wound infection, a total of 24 patients were included in the study. The patients were evaluated of disease severity, revision surgical method, time interval to recurrence, underlying diseases and postoperative functional outcomes.

RESULTS

All patients received ulnar nerve subfascial anterior transposition for the initial cubital tunnel syndrome surgery. Among the 24 patients (3.7%) who received revision surgery, nine received in situ neurolysis, 12 received submuscular transposition, and three received subcutaneous transfer. 21 patients (88%) reported improved mean VAS score of 4.3, while three patients complained of remaining symptoms that did not improve even after revision surgery. The patients with remaining symptoms all had underlying diabetes mellitus and were treated with subcutaneous transfer. The difference of surgical outcomes between the in situ neurolysis group and the nerve transfer groups were non-significant (p = 0.23). The most common cause of recurrent or persistent symptoms was adhesion and fibrosis at sling area. The mean follow up period before revision surgery was 26.3 months and postoperative follow up period after revision surgery was 8.5 months.

CONCLUSION

The outcomes of this study imply that in-situ neurolysis may be as effective as anterior submuscular transfer of ulnar nerve for refractory cubital tunnel syndrome after anterior subfascial transfer.

From Conservative Measures to Surgical Interventions, Treatment Approaches for Cubital Tunnel Syndrome: A Comprehensive Review

Cubital tunnel syndrome (CuTS) is a neuropathic condition characterized by the compression or irritation of the ulnar nerve at the elbow, resulting in a wide spectrum of symptoms ranging from pain and numbness to muscle weakness and impaired hand function. This comprehensive review delves into the diverse landscape of CuTS treatment approaches, emphasizing the importance of early intervention. The review explores how these strategies aim to alleviate symptoms and enhance patient well-being by beginning with conservative measures encompassing rest, splinting, medications, physical therapy, and lifestyle adjustments. Non-surgical medical interventions, including nerve gliding exercises, ultrasound-guided nerve injections, and orthotic devices, are considered alternative therapies for symptom relief. Surgical interventions, such as decompression procedures and emerging techniques, are discussed in detail, highlighting their indications and expected outcomes. Throughout this review, the critical role of patient-centered care is underscored, emphasizing the need for tailored treatment plans that respect individual preferences and goals. Recognizing the unique nature of each CuTS case, shared decision-making between patients and healthcare providers is advocated, ensuring that interventions align with specific patient needs. As research advances, promising developments in diagnosis, surgical techniques, and drug therapies offer hope for more effective management of CuTS, paving the way for improved symptom relief and enhanced nerve function.

Ulnar nerve anteposition with adipofascial flap, an alternative treatment for severe cubital syndrome

BACKGROUND

Ulnar nerve entrapment at the elbow is the second most common cause of nerve entrapment in the upper limb. Surgical techniques mainly include simple decompression, decompression with anterior transposition and medial epicondylectomy.

METHODS

We performed decompression with anterior transposition and protected ulnar nerve by adipofascial flap (a random flap with radial based vascularization, harvested through the avascular plane of Scarpa's fascia. We analyzed patients who underwent ulnar nerve ante-position from 2015 to 2022 according to inclusion and exclusion criteria for a total of 57 patients. All patients included were graded on the McGowan's classification Messina criteria and the British Medical Research Council modified by Mackinnon and Dellon.

RESULTS

The average McGowan's score was 2.4 (± 0.6), Messina's criteria 91.2% indicated a satisfactory or excellent result, sensibility at 6 months was 98.5% S3 or more. A preferential technique has not yet been defined.

CONCLUSIONS

The adipofascial flap offers numerous advantages in providing a pliable, vascular fat envelope, which mimics the natural fatty environment of peripheral nerves and creates favorable micro-environmental conditions to contribute to neural regeneration via axon outgrowth.

The modified fascial sling technique for ulnar nerve anterior transposition: surgical techniques and results

Background

Approaches to surgical treatment to cubital tunnel syndrome include simple decompression, decompression with medial epicondylectomy, and decompression with anterior transposition of the ulnar nerve. Transposition of the ulnar nerve involves decompression and transposition of the nerve anteriorly to a subcutaneous, intramuscular, or submuscular position. However, transposing the ulnar nerve to subcutaneous plane renders it more susceptible to external trauma. Hence, this technique article introduces the use of a modified fascial sling.

Methodology

The modified fascial sling technique for anterior transposition of the ulnar nerve involves careful dissection to identify the ulnar nerve, decompression of the nerve, then transposition of the ulnar nerve anterior to the medial epicondyle. An AlloWrap (Stryker, Kalamazoo, MI, USA) is first wrapped around the ulnar nerve, followed by wrapping a fascial sling fashioned from the flexor carpi ulnaris fascia. A prospective case series for this surgical technique was conducted. Wilcoxon signed-rank test compared preoperative and postoperative qDASH-9 scores, an abbreviated questionnaire to assess functional limitations of the upper limb.

Results

Five patients were included in this study, with a mean duration of follow-up of 530.4 days. The mean QuickDASH-9 functional disability score was 36.5 ± 25.1 preoperatively and 20.6 ± 12.8 postoperatively, demonstrating statistically significant improvement (P = .008).

Conclusion

The modified fascial sling technique for anterior transposition of the ulnar nerve was developed to address the complications of perineural adhesions after transposition causing tethering of the ulnar nerve. At the same time, the fascial sling prevents posterior subluxation of the ulnar nerve back to its original location, thereby reducing the risk of recurrent symptoms.

A complete analysis of the surgical treatment for cubital tunnel syndrome: an umbrella review

BACKGROUND

Numerous original studies have been presented regarding various surgical treatments for cubital tunnel syndrome (CubTS). The results of these analyses regarding the different surgical treatments for CubTS frequently differ and, in some cases, contradict each other, creating confusion. Therefore, our umbrella review aimed to summarize the evidence from meta-analyses and systematic reviews regarding the surgical treatments of CubTS, and along with the new, more detailed outcomes, provide an evidence-based tool for surgeons performing these procedures.

MATERIALS AND METHODS

To perform this umbrella review, a systematic search was conducted for meta-analyses investigating CubTS in PubMed, Scopus, Embase, Web of Science, Google Scholar, Cochrane Library, BIOSIS, and EBSCO medical databases.

RESULTS

Detailed comparison of surgical outcomes, such as clinical improvement, complications, or reoperation rates, of different treatment methods for CubTS was performed. New risk ratios in all categories were established based on all of the primary studies available in the literature.

DISCUSSION

The goal of the present umbrella review was to gather and summarize all data about the surgical treatments for CubTS in the available literature and provide new outcomes and an evidence-based tool for surgeons performing these procedures. Each comparison of the surgical techniques has been discussed, and the present study's results were compared with the outcomes of the previous studies.

Factors influencing return to work after surgery for ulnar nerve compression at the elbow

Ulnar nerve compression at the elbow (UNE) frequently affects people of working age. Surgically treated patients may not immediately return to work (RTW) postoperatively. In 2008, the Swedish Social Insurance Agency reformed the national insurance policy. We aimed to examine RTW postoperatively for UNE, variations among surgical methods, and potential risk factors for prolonged RTW (sick leave > 6 weeks). Surgically treated cases of UNE (n = 635) from two time periods (2004-2008 and 2009-2014) and two healthcare regions (Southern and South-eastern) were studied retrospectively regarding age, sex, comorbidities, occupation, type of surgery and time to RTW. A sub-analysis of the exact number of weeks before RTW (n = 201) revealed longer RTW for unemployed cases compared to employed cases. Prolonged RTW was seen among younger, manual workers and after transposition or revision surgery. Prolonged RTW was approximately four times more likely after transposition than after simple decompression. Comparisons before and after 2008 showed occupational differences and differences in RTW, where cases operated before 2008 more often had permanent sickness benefit, but the reform of the social insurance system did not seem to influence RTW. In conclusion, unemployment, younger age at surgery, manual labour, transposition, and revision surgery were related to prolonged RTW.

Cubital Tunnel Syndrome: Review of Diagnosis and Management

Cubital tunnel syndrome is one of the most common upper extremity nerve entrapment conditions. Clinical examination and electrodiagnostic studies assist in the diagnosis. Mild cases can be successfully treated conservatively, aiming to reduce traction and compression on the nerve. Surgical management of cubital tunnel syndrome is increasing in frequency. Multiple surgical options exist ranging from simple decompression to decompression and anterior transposition of the nerve. There is no preferred surgical technique. It is critical to reduce the risk of recurrent cubital tunnel syndrome. Revision surgical procedures have worse outcomes than primary surgical procedures.

Submuscular transposition of the ulnar nerve for persistent or recurrent cubital tunnel syndrome: Results of a prospective case series

BACKGROUND

Submuscular transposition (SMT) of the ulnar nerve is frequently performed as secondary procedure in patients with persistent or recurrent cubital tunnel syndrome (CuTS) despite previous surgery. Good results have been reported for this surgical strategy, but mainly in small retrospective case series. The goal of the present study is therefore to analyze the results prospectively using a patient-reported outcome measure (PROM): patient-rated ulnar nerve evaluation (PRUNE).

METHODS

SMT of the ulnar nerve was performed in 30 consecutive patients who were referred because of persistent or recurrent CuTS despite previous surgery. Objective outcome was measured in the outpatient clinic using the Likert scale. The PRUNE questionnaire was obtained pre-operatively, 6 weeks, 3 months, 6 months, and 12 months after the surgery. Simultaneously, 20 patients with primary surgery for CuTS, that underwent simple decompression, were followed.

RESULTS

Good outcome (Likert 1 or 2) was obtained in 67% after SMT for persistent/recurrent CuTS and 85% after decompression as primary surgical treatment. PRUNE scores were significantly decreased in both groups at all follow-up moments after surgery compared with pre-operative for the total questionnaire and subscales "pain," "sensory/motor symptoms," and "specific activities." In both groups, PRUNE score remained stable until 12 months of follow-up.

CONCLUSION

This prospective study confirms previous results from retrospective studies showing that SMT is an effective surgical option for persistent or recurrent CuTS. Prospective (randomized controlled) trials are needed to compare the effectiveness of SMT to the surgical alternative of subcutaneous transposition of the ulnar nerve.

Ulnar nerve dislocation in ulnar nerve entrapment at the elbow. Influence on surgical outcome

Our aim was to assess the incidence of symptomatic ulnar nerve dislocation and its influence on surgical outcome after primary and revision surgeries in ulnar nerve entrapment at the elbow (ulnar neuropathy at the elbow (UNE) or cubital tunnel syndrome). The influence of pre- or intra-operative ulnar nerve dislocation on postoperative outcome was assessed in 548 surgically treated cases (548 nerves) from two hand surgery departments reporting to the Swedish National Quality Registry for Hand Surgery, using QuickDASH, a patient-reported outcome measure (PROM), before surgery and at 3 and 12 months postoperatively, and a doctor-reported outcome measure (DROM), grading as "cured-improved "or "unchanged-worsened," at a median follow-up of 3.0 months [IQR, 1.5-6.0]. 109 of the 548 cases (20%) showed documented pre- or intra-operative ulnar nerve dislocation; more often found at revision (35/75, 47%) than at primary surgery (74/473, 16%) (p < 0.0001). Cases with dislocation presented higher QuickDASH scores at 12 months (p = 0.026). A linear regression model, adjusted for age and gender, predicted higher QuickDASH scores at 12 months postoperatively for cases with dislocation (unstandardized B 11.3 [95% CI 0.4-22.2], p = 0.043). DROM grading as unchanged-worsened at a median 3 months predicted worse QuickDASH scores (p < 0.0001) than in cured-improved cases at 3 (unstandardized B, 18.4 [95% CI 9.4-27.3]) and 12 months (unstandardized B, 18.1 [9.1-27.0]). Primary surgeries had better DROM grading than revision surgeries (p = 0.033; cured-improved, 75% and 63%, respectively), but QuickDASH scores did not differ. Presence of a clinically relevant ulnar nerve dislocation resulted in worse outcome, perhaps due to more extensive surgery with transposition. Nerve dislocation needs attention when treating UNE patients.

Management of Recalcitrant Cubital Tunnel Syndrome

Cubital tunnel syndrome is a common upper extremity compressive neuropathy. Recalcitrant cubital tunnel syndrome poses diagnostic and treatment challenges. Potential etiologies of persistent or recurrent symptoms after surgical treatment include an inaccurate preoperative diagnosis, incomplete nerve decompression, iatrogenic injury, postsurgical perineural adhesions, irreversible nerve pathology, or conditions associated with secondary nerve compression. Confirmation of recalcitrant ulnar nerve pathology relies on a thorough history to consider symptoms and chronology, careful examination to quantify nerve function and to assess for focal nerve provocation, and objective testing to highlight a possible nerve lesion such as ultrasonography and electrodiagnostic testing. Conservative treatment may provide symptomatic relief; however, surgical management such as revision neuroplasty, neurolysis, nerve reconstruction, and/or anterior transposition may be indicated. Optimizing the biology of the local nerve environment is critical. No surgical treatment procedure has shown superiority over another; however, individualized treatment is emphasized to improve symptoms and maximize nerve recovery potential.

Ulnar Neuropathy at the Elbow: From Ultrasound Scanning to Treatment

Ulnar neuropathy at the elbow (UNE) is commonly encountered in clinical practice. It results from either static or dynamic compression of the ulnar nerve. While the retroepicondylar groove and its surrounding structures are quite superficial, the use of ultrasound (US) imaging is associated with the following advantages: (1) an excellent spatial resolution allows a detailed morphological assessment of the ulnar nerve and adjacent structures, (2) dynamic imaging represents the gold standard for assessing the ulnar nerve stability in the retroepicondylar groove during flexion/extension, and (3) US guidance bears the capability of increasing the accuracy and safety of injections. This review aims to illustrate the ulnar nerve's detailed anatomy at the elbow using cadaveric images to understand better both static and dynamic imaging of the ulnar nerve around the elbow. Pathologies covering ulnar nerve instability, idiopathic cubital tunnel syndrome, space-occupying lesions (e.g., ganglion, heterotopic ossification, aberrant veins, and anconeus epitrochlearis muscle) are presented. Additionally, the authors also exemplify the scientific evidence from the literature supporting the proposition that US guidance is beneficial in injection therapy of UNE. The non-surgical management description covers activity modifications, splinting, neuromobilization/gliding exercise, and physical agents. In the operative treatment description, an emphasis is put on two commonly used approaches-in situ decompression and anterior transpositions.

Ultrasound-Assisted Precise In Situ Decompression for Cubital Tunnel Syndrome

Objective

To explore the effect of locating the ulnar nerve compression sites and guiding the small incision so as to decompress the ulnar nerve in situ on the elbow by high‐frequency ultrasound before operation.

Methods

A retrospective analysis was conducted on 56 patients who underwent ultrasound‐assisted in situ decompression for cubital tunnel syndrome from May 2018 to August 2019. The patients' average age was 51.13 ± 7.35 years, mean duration of symptoms was 6.51 ± 1.96 months, and mean postoperative follow‐up was 6.07 ± 0.82 months. Nine patients had Dellon's stage mild, 39 had stage moderate, and eight had stage severe. Ultrasound and electromyography were completed in all patients before operation. The presence of ulnar nerve compressive lesion, the specific location, and the reason and extent of compression were determined by ultrasound. A small incision in situ surgery was given to decompress the ulnar nerve according to the pre‐defined compressive sites.

Results

All patients underwent in situ decompression. The compression sites around the elbow were as follows: two in the arcade of Struthers, one in the medial intermuscular septum, four in the anconeus epitrochlearis muscle, five beside the cyst of the proximal flexor carpi ulnaris (FCU), and the remaining 44 cases were all from the compression between Osborne's ligament to the two heads of the FCU. The compression localizations diagnosed by ultrasound were confirmed by operations. Preoperative ultrasound confirmed no ulnar nerve subluxation in all cases. The postoperative outcomes were satisfactory. There was no recurrence or aggravation of symptoms in this group of patients according to the modified Bishop scoring system; results showed that 43 cases were excellent, 10 were good, and three were fair.

Conclusions

High‐frequency ultrasound can accurately and comprehensively evaluate the ulnar nerve compression and the surrounding tissues, thus providing significant guidance for the precise minimally invasive treatment of ulnar nerve compression.

Challenging the dogma: anterior transposition of the ulnar nerve is indicated in recurrent cubital tunnel syndrome

The current evidence for treatment of primary idiopathic cubital tunnel syndrome favours an in situ release. However, anterior transposition of the ulnar nerve remains a popular procedure in recurrent cubital tunnel syndrome. For more than 20 years, I have performed an extended in situ release only, and achieved similar or better results than with nerve transposition. In performing a systematic review of the evidence for surgery for recurrent cubital tunnel syndrome, I could only include 16 out of 296 studies regarding treatment of recurrent cases of cubital tunnel syndrome. A meta-analysis was not possible, due to selection bias and disparity of outcome measurements of the studies. However, I could not find robust evidence that supports the need of an anterior transposition of the ulnar nerve in recurrent cubital tunnel syndrome over an in situ decompression. My own experience of an extended in situ release with complete neurolysis with reasonable outcomes, in combination with the lack of literature support of anterior transposition in recurrent cases, have led me to the consideration that this dogma should be revised.

The impact of pre-existing ulnar nerve instability on the surgical treatment of cubital tunnel syndrome: a systematic review

BACKGROUND

The decision to perform nerve transposition (NT) or in situ decompression (SD) during surgical treatment of cubital tunnel syndrome is often based on nerve subluxation through elbow motion. This review assesses what impact nerve instability has on study design and reported outcomes.

METHODS

A search was performed with Boolean operators: "ulnar nerve" OR "cubital tunnel" AND "decompression" OR "transposition" on PubMed, Clinical Key, and CINAHL to identify primary studies comparing NT and SD that report pre-existing nerve instability. Primary outcome was the effect of instability on study design. Secondary outcomes were nerve instability, patient-reported scores, and complications.

RESULTS

Five studies met criteria after screening 134 articles. In 3 studies, nerve instability dictated treatment. Prospective randomization was maintained in 1 study. Included cases totaled 464 SD and 304 NT. The complication rate was 8.6% overall, 4.3% for SD and 21.1% for NT. Bishop scores were 56.9% excellent and 37.3% good for stable nerves and 62.0% excellent and 29.3% good for unstable nerves.

CONCLUSIONS

Very few studies report ulnar nerve instability, and study design is biased by ulnar nerve subluxation. Outcomes showed similar symptomatic improvement for both decompressed and transposed groups with higher complication rates for the transposed group.

Cubital Tunnel Syndrome: Current Concepts

PURPOSE OF REVIEW

Compressive neuropathy of the ulnar nerve across the elbow is a common diagnosis encountered frequently within a hand and upper extremity clinical practice. Appropriate and timely evaluation, diagnosis, objective testing, and evidence-based decisions regarding treatment options are paramount in the optimal care of the patient with this pathology. An understanding of current literature is critical in determining and understanding best practices.

RECENT FINDINGS

A thorough review of the recent literature regarding physical examination, diagnostic testing, and nonoperative versus operative results was performed. Regarding physical examination, the glenohumeral internal rotation test and scratch collapse test are more effective and sensitive than traditional maneuvers such as Tinel's testing and the elbow flexion test. Electrodiagnostic testing, magnetic resonance imaging, and ultrasound evaluation have all been shown to be effective in diagnosing cubital tunnel syndrome. However, no single test has proven itself to be superior. Nonoperative treatment can be successful for mild cases of cubital tunnel syndrome. Surgical release techniques comparing open with endoscopic release are equivocal, and in situ release versus transposition techniques show that transposition should not be performed routinely. The diagnosis and treatment of cubital tunnel syndrome do not have a well-defined algorithm based on current literature. The treating physician must therefore utilize the available information to determine a diagnostic and treatment plan individualized to the patient. More rigorous scientific studies are needed to determine the most effective surgical approaches for cubital tunnel syndrome.

Patient-reported outcomes after in situ cubital tunnel decompression: a report in 77 patients

This prospective single-centre study describes the patient-reported outcomes following open in situ decompression for cubital tunnel syndrome. The Quick version of the Disabilities of the Arm, Shoulder and Hand questionnaire and patient satisfaction scores were collected over a 3-year period. Outcomes were available for 77 patients at a mean of 17 months (11-27 months) postoperatively. The mean scores improved significantly from 39 to 30 postoperatively. The score change correlated strongly and significantly with postoperative satisfaction. Sixty-six patients (86%) were satisfied. Patients with clinical evidence of weakness at presentation had significantly lower satisfaction scores than those without. By reporting functional improvement and high levels of patient satisfaction in a large series, this study supports the use of in situ decompression for cubital tunnel syndrome. The outcomes for the patients with weakness and atrophy of ulnar nerve innervated muscles before surgery are less satisfactory. Level of evidence: III.

Patient Characteristics in Ulnar Nerve Compression at the Elbow at a Tertiary Referral Hospital and Predictive Factors for Outcomes of Simple Decompression versus Subcutaneous Transposition of the Ulnar Nerve

Patient characteristics and predictive factors for outcomes were analysed in 202 cases undergoing simple decompression, primary subcutaneous transposition, or secondary subcutaneous transposition for ulnar nerve compression at the elbow at a tertiary referral hospital. Data from medical charts and a survey were evaluated. The mean patient age was 49 years with revision surgery cases being significantly younger. Sixty-one percent of cases were female, and 31% were smokers. The comorbidity was extensive, including other nerve compression lesions as well as neck and shoulder problems. Overall, 53% reported being pleased with the result of surgery and 57% of the cases rated function as better or completely recovered after surgery. The median postoperative DASH (Disabilities of the Arm, Shoulder and Hand) score was 26 (IQR 11–49), which is in accordance with unpublished national data. No significant differences in DASH scores were found between surgical groups, but a higher preoperative McGowan grade was significantly associated with a poorer postoperative DASH score. Women scored greater disability postoperatively than men. There was a significantly increased risk of complications, which was doubled for smokers, following primary and secondary transposition compared to simple decompression. Surgical cases with ulnar nerve compression treated at a tertiary referral hospital constitute a heterogeneous group with great comorbidity and frequent concomitant nerve compression lesions. We suggest simple decompression as the procedure of first choice. Transposition can be used in selected cases or when simple decompression fails. All patients should be strongly recommended to stop smoking considering the remarkably increased risk for complications among smokers.