Steven J. McCabe

The seven deadly sins of statistical analysis

In a pedantic but playful way, we discuss some common errors in the use of statistical analysis that are regularly observed in our professional plastic surgical literature. The seven errors we discuss are (1) the use of parametric analysis of ordinal data; (2) the inappropriate use of parametric analysis in general; (3) the failure to consider the possibility of committing type II statistical error; (4) the use of unmodified t-tests for multiple comparisons; (5) the failure to employ analysis of covariance, multivariate regression, nonlinear regression, and logistical regression when indicated; (6) the habit of reporting standard error instead of standard deviation; and (7) the underuse or overuse of statistical consultation. Confidence and common sense are advocated as a means to balance statistical significance with clinical importance.

Vascular isolation of the rat cremaster muscle

The vascular supply to the rat cremaster muscle was completely isolated to provide a microvascular preparation with a controllable blood flow. The anatomy of the cremaster vascular supply and the surgical approach to isolate the cremaster muscle on its neurovascular pedicle are described. The functional integrity of this isolated cremaster preparation was tested using intravital video microscopy to evaluate the tissues response to vasoactive agents and to peripheral nerve stimulation. The isolated cremaster muscle was positioned in situ in a controlled tissue bath and concentration response curves to the topical application of norepinephrine (NE) and acetylcholine (Ach) were determined. Vasoconstriction elicited by the topical application of NE or by stimulation of the genitofemoral nerve trunk was similar for both the isolated and standard cremaster preparations. Application of 10(-5) M Ach caused maximal vasodilation equal to that produced by 10(-3) M papaverine in both preparations. In summary, the isolated cremaster muscle of the rat is an acceptable model of a skeletal muscle microcirculation which can be used to investigate microvascular function when precise monitoring or control of perfusion to the entire muscle is needed.

The measurement of cold sensitivity of the hand

There is no widely accepted method of measuring the severity of cold sensitivity of the hand. A patient-answered questionnaire was developed. This consisted of seven items divided into two subscales: the first to measure the severity of cold sensitivity, and the second to grade exposure of the hands to cold at the workplace. The test-retest reliability was 0.92 for the severity subscale and 0.94 for the work exposure scale. An initial test of validity supports the use of the scale to measure cold sensitivity of the hand.

Direct in Vivo Observations of Embolic Events in the Microcirculation Distal to a Small-vessel Anastomosis

This study was done to determine whether microemboli are produced by an arterial anastomosis. Direct in vivo observations were made in an isolated microcirculatory bed lying directly downstream from a newly made anastomosis. The tissue used was the isolated rat cremaster muscle, a new experimental model. The vessel anastomosed was the external iliac artery. Following anastomosis, microemboli were clearly observed in eight of eight animals during the first 30 minutes after clamp release. Embolic events were sometimes of impressive magnitude and in one case were associated with cessation of blood flow throughout the preparation. No microemboli were observed in eight of eight animals subjected only to dissection of the cremaster, nor were any observed in eight of eight animals in which the isolated cremaster was subjected only to 2 hours of clamp ischemia. These findings may be significant in explaining perturbations to blood flow following free-tissue transfer and instances of partial tissue necrosis following apparently successful arterial repair. These findings also identify an important factor (microemboli) to be considered in research on reperfusion injury.

Verification of the Pressure Provocative Test in Carpal Tunnel Syndrome

Three provocative tests (pressure, Phalens test, and Tinels sign) were studied in 30 patients with carpal tunnel syndrome and 30 control subjects. The pressure provocative test had a sensitivity of 100%. In contrast, Phalens test was 88% sensitive and Tinels sign only 67% sensitive. The pressure provocative test is a sensitive indicator of median nerve compression at the wrist with a faster reaction time than Phalens test (mean time of 9 seconds vs 30 seconds). It is an appropriate provocative test in patients with stiff or painful wrists when wrist flexion is restricted.Williams TM, Mackinnon SE, Novak CB, McCabe S, Kelly L: Verification of the pressure provocative test in carpal tunnel syndrome. Ann Plast Surg 1992;29:8–11

Nonunion rates of limited carpal arthrodesis: A meta-analysis of the literature*

Numerous types of limited intercarpal arthrodeses have been reported in dozens of articles in the English-language medical literature. The nonunion rate varies considerably within and between the different types of arthrodeses. This may be due in part to the small number of cases in most studies. The data on the number of good results and nonunion rates for 27 different types of limited carpal arthrodeses reported in the English-language medical literature from 1946 to 1993 were collected, and for each study, the 95% confidence intervals (95% CI) were calculated. The studies were then combined for specific types of arthrodeses and the overall nonunion rates and confidence intervals were calculated to more accurately determine the actual nonunion rate expected for limited intercarpal arthrodeses. Nonunion rates (95% CI) for the most popular types of limited arthrodeses were as follows: of 385 scaphotrapezium-trapezoid arthrodeses reported, there was a 14% nonunion rate (95% CI, 11%-18%); of 104 lunotriquetral arthrodeses, the nonunion rate was 27% (95% CI, 19%-36%); and of 17 scapholunate arthrodeses, the nonunion rate was 47% (95% CI, 26%-69%).

Kienböck disease and negative ulnar variance.

We compared the degree of ulnar variance, measured on standardized radiographs of the wrist, in forty-four patients who had Kienböck disease with that in ninety-nine control subjects who had been selected from a general clinic population and had radiographs of the wrist. The purpose of our study was to determine if there is a true relationship between negative ulnar variance and the development of Kienböck disease. Gender was not found to influence the degree of ulnar variance, but an association was found between age and negative ulnar variance in both the control subjects and the patients who had Kienböck disease. The findings of the present study confirmed an association between negative ulnar variance and the development of Kienböck disease even after correction for the influence of age on the measurement of ulnar variance.

Determining the sensitivity and specificity of common diagnostic tests for carpal tunnel syndrome using latent class analysis.

Background: The accuracy of a diagnostic test used to classify a patient as having disease or being disease-free is a valuable piece of information to be used by the physician when making treatment decisions. If a standard reference test is available, determining the sensitivity and specificity of a new test is straightforward. However, if that reference test is incorrectly assumed to be perfectly sensitive and specific, the errors of the reference test can result in an underestimation of the accuracy of the test being evaluated. Latent class analysis can be applied to determine the sensitivity and specificity of a new test when no standard exists. Methods: Latent class analysis was used to determine the accuracy rates of three commonly used measures of carpal tunnel syndrome: Tinel’s sign, Phalen’s test, and the nerve conduction velocity test. Data included 162 wrists from 81 patients seeking treatment for symptoms associated with carpal tunnel syndrome. Results: Tinel’s sign and Phalen’s test were both highly sensitive (0.97 and 0.92, respectively) and specific (0.91 and 0.88, respectively). The sensitivity and specificity of the nerve conduction velocity test were 0.93 and 0.87, respectively. Conclusion: Estimates of the sensitivity and specificity of these common tests for carpal tunnel syndrome support their widespread clinical use.

Assessment of the carpal tunnel outcome instrument in patients with nerve-compression symptoms

The outcome movement in medicine has encouraged the development of patient-answered questionnaires as measures of well-being. A disease-specific questionnaire for carpal tunnel syndrome (CTS) was introduced by Levine et al. in 1993. We evaluated this questionnaire in 156 consecutive new patients presenting with pain, numbness, or tingling of the upper extremity. Of these, 114 correctly filled out the carpal tunnel outcome instrument. In addition, these patients completed the self-administered hand diagram developed by Katz and Stirrat for the diagnosis of CTS. The 114 patients were classified according to their hand diagram as classic or probable CTS (n = 47), possible CTS (n = 31), and unlikely CTS (n = 36). The mean symptom severity score in patients classified as classic or probable CTS was significantly higher than the mean score in patients classified as possible or unlikely CTS (p < .01). The mean scores of items regarding sensory symptoms were significantly higher in patients with classic or probable CTS compared to patients with possible or unlikely CTS (p < .0001). The scores were similar for CTS and non-CTS patients on the functional status subscale.

The nerve of Henlé

The nerve of Henlé, a branch of the ulnar nerve in the forearm, is thought to deliver sympathetic innervation to the ulnar artery. Forty cadaver forearms were dissected under magnification. Two distinct patterns of the nerve were found. In the typical pattern, 18 (45%) of 40 extremities, the nerve originates 16 cm proximal to the ulnar styloid, travels distally with the ulnar artery, and frequently, 13 (72%) of 18, branches to pierce the superficial fascia 6 cm proximal to the ulnar styloid and innervate the skin of the distal ulnar forearm. In the atypical pattern (12%), the nerve originates in the distal 8 cm of the forearm and travels briefly with the ulnar artery before branching to the skin. The palmar cutaneous branch of the ulnar nerve was absent in cadavers with the nerve of Henlé and may be a distal variant of that nerve.