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The Parallelogram Effect: The Association Between Central Band and Positive Ulnar Variance

Open AccessPublished:May 24, 2018DOI:https://doi.org/10.1016/j.jhsa.2018.03.058

      Purpose

      Ulnar impaction syndrome is a poorly understood degenerative wrist condition characterized by symptoms of pain thought to be caused by increased loads between the ulnar head and the carpals. Radiographic evaluation often reveals an ulnar-positive wrist. We hypothesize that progressive elongation of the central band of the forearm interosseous ligaments changes the longitudinal radial–ulnar relationships, resulting in an ulnar-positive wrist. The objective of the study was to identify a relationship between the loss of integrity of the forearm interosseous ligaments and increased ulnar variance.

      Methods

      Six cadaveric human forearms were used to measure displacement of the radius relative to the ulna during axial loading of the lunate fossa of the radius. Radial heights were measured in supination and pronation under a 5-lbF (22-N) preload. Gradual axial loads were applied up to 50 lbF (222N); the resultant axial displacement was measured in supination and pronation. All measurements were evaluated with the interosseous ligament intact and repeated with the central band cut.

      Results

      With an applied 5-lbF preload, cutting the central band increased ulnar variance by 3.02 ± 0.80 mm in supination and by 2.15 ± 0.79 mm in pronation. In supination, when the loads were increased from the 5-lbF preload to 50 lbF, the radius displaced 2.1 times further after the central band was cut (3.00 mm) compared with the group with the intact forearm construct (1.41 mm). In pronation, when the loads were increased from the 5-lbF preload to 50 lbF, the radius displaced 1.8 times further when the central band was cut (2.84 mm) than with the intact forearm construct (1.57 mm).

      Conclusions

      Because of a parallelogram effect, the radius shifted proximally under a 5-lbF preload, creating an ulnar-positive wrist relationship. Dynamic loading of the forearm after ligament excision resulted in significant additional radial displacement relative to the intact forearm.

      Clinical relevance

      Deficiency in the ligamentous restraints of the central band leads to positive ulnar variance, which could be a factor (among others) that contributes to idiopathic ulnar impaction syndrome.

      Key words

      Positive ulnar variance can be associated with ulnar-sided wrist pain, lunate erosion, or both. The radiographic appearance of ulnar variance depends on the degree of forearm rotation; variance increases in forearm pronation and decreases in forearm supination (relative to neutral forearm rotation).
      • Friedman S.L.
      • Palmer A.K.
      The ulnar impaction syndrome.
      • Epner R.A.
      • Bowers W.H.
      • Guilford W.B.
      Ulnar variance: the effect of wrist positioning and roentgen filming technique.
      • Escobedo E.M.
      • Bergman A.G.
      • Hunter J.C.
      MR imaging of ulnar impaction.
      • Palmer A.K.
      • Glisson R.R.
      • Werner F.W.
      Ulnar variance determination.
      In addition, ulnar variance has been shown to increase significantly during grip.
      • Friedman S.L.
      • Palmer A.K.
      • Short W.H.
      • Levinsohn E.M.
      • Halperin L.S.
      The change in ulnar variance with grip.
      • Tomaino M.M.
      The importance of the pronated grip x-ray view in evaluating ulnar variance.
      Notably, previous cross-sectional studies showed an association between positive ulnar variance and age, with positive ulnar variance being associated with increasing age;
      • Bonzar M.
      • Firrell J.C.
      • Hainer M.
      • Mah E.T.
      • McCabe S.J.
      Kienböck disease and negative ulnar variance.
      • Sanderson P.L.
      • Cameron I.C.
      • Stanley D.
      Ulnar variance and age.
      this is an unexpected finding because the bones should not change in length after the growth plates have closed.
      Alignment of the upper-extremity skeleton is maintained by ligamentous structures stabilizing the joints among the humerus, radius, and ulna;
      • Farr L.D.
      • Werner F.W.
      • McGrattan M.L.
      • Zwerling S.R.
      • Harley B.J.
      Anatomy and biomechanics of the forearm interosseous membrane.
      these form a closed kinetic chain. The longitudinal axis of the arm and forearm form a small valgus angle. In supination, this angle is defined as the carrying angle, which normally varies between 5° and 15° and is slightly larger in women than in men. Longitudinal stability between the radius and ulna is provided by the forearm interosseous ligament system (Fig. 1). This ligamentous system consists of 3 parts: the proximal and distal oblique ligaments, which prevent distal displacement of the radius relative to the ulna, and the central band, which prevents proximal displacement of the radius relative to the ulna. The central band originates distally from the ulna and inserts proximally on the radius. It is aligned approximately 21° to the longitudinal axis of the forearm.
      • Skahen J.R.
      • Palmer A.K.
      • Werner F.W.
      • Fortino M.D.
      Reconstruction of the interosseous membrane of the forearm in cadavers.
      • Orbay J.L.
      • Mijares M.R.
      • Berriz C.G.
      The transverse force experienced by the radial head during axial loading of the forearm: a cadaveric study.
      This arrangement allows the central band to transfer axial loads from the radius to the ulna.
      • Birkbeck D.P.
      • Failla J.M.
      • Hoshaw S.J.
      • Fyhrie D.P.
      • Schaffler M.
      The interosseous membrane affects load distribution in the forearm.
      • Majima M.
      • Horii E.
      • Matsuki H.
      • Hirata H.
      • Genda E.
      Load transmission through the wrist in the extended position.
      Figure thumbnail gr1
      Figure 1The interosseous ligament is composed of A proximal, B central, and C distal oblique bands. The central band originated distally from the ulna and inserts proximally on the radius and is aligned approximately 21° to the longitudinal axis of the forearm.
      The objective of this study was to evaluate the relationship between a loss of integrity of the forearm interosseous ligaments and ulnar variance. We hypothesized that loss of integrity of the central band of the forearm interosseous ligaments may result in ulnar-positive variance. We also hypothesized that failure of the central band would result in proximal displacement of the radius with respect to the ulna and that both bones would progressively shift into a less valgus alignment simultaneously until a new position of stability was reached.

      Materials and Methods

      Specimen preparation

      A sample of convenience composed of 6 cadaveric human forearms (average age, 52.6 years; 3 males and 3 females) was used to measure displacement experienced by the radius relative to the ulna during axial loading of the radius. Specimens were evaluated radiographically with the wrist in neutral forearm rotation and the elbow flexed 90° (positioned in neutral with an unloaded wrist) for signs of wrist pathology. None of the specimens showed signs of wrist pathology or ulnar-positive alignment in the neutral position.
      The skin, muscles, tendons, vasculature, and nerves were discarded. The interosseous ligament complex, triangular fibrocartilage complex, distal and proximal radioulnar joint ligaments, and elbow collateral ligaments were preserved. The humerus was cut midshaft and the carpals were removed. The ulnar shaft was oriented vertically and rigidly fixed to an adjacent vertical beam with 2 screws. The forearm construct was then statically positioned in a Mark-10 test stand (version ESM301L, Copiague, NY) in both supination and pronation for testing (Fig. 2).
      Figure thumbnail gr2
      Figure 2The experimental setup. The ulna is rigidly attached to a vertical beam. Load is applied to the distal radius by a Mark-10 test stand in both supination and pronation.

      Ulnar variance caused by physiological loading

      The Mark-10 test stand was used to apply axial loads to the articular surface of the lunate fossa of the distal radius (25.4 mm/min). As the radius displaced, the resistive loads increased. A 5-lbF preload was applied to simulate baseline muscle tone. The position of the distal edge of the radius (radial position within the test stand) was noted under a 5-lbF preload in supination and pronation.
      For the purposes of this study, the radial position noted under a 5-lbF preload with intact interosseous ligaments was defined as the specimen’s baseline position (physiological position). Clinically, ulnar variance refers to the height difference between the distal articular surfaces of the radius and ulna. For the purposes of this study, change in ulnar variance refers to the axial displacement of the radius relative to the specimen’s baseline position.

      Ulnar variance caused by grip loading

      The Mark-10 test stand was used to apply axial loads gradually (25.4 mm/min) from the 5-lbF preload up to 50 lbF, and the resultant axial displacement (change in ulnar variance) was measured in supination and pronation. The 50 lbF measurement was intended to simulate active power grip. All measurements were evaluated with the interosseous ligament intact and repeated with the central band of the interosseous ligament cut and completely excised. Cutting the central band of the interosseous ligament represented acute failure of the central band. Loading and measurements were repeated 5 times for each specimen in each of the 4 loading configurations.

      Statistical analysis

      We used a 2-tailed paired t test to compare the radial displacement that occurred when loading was increased from the 5-lbF preload to 50 lbF of simulated grip intact versus forearms with the central band cut in each loading configuration. P ≤ .01 was considered significant.

      Results

      Ulnar variance caused by physiological loading

      With an applied 5-lbF preload, cutting the central band of the interosseous ligament increased ulnar variance by 3.02 ± 0.80 mm in supination and by 2.15 ± 0.79 mm in pronation.

      Ulnar variance caused by grip loading

      In supination, when the loads were increased from the 5-lbF preload to 50 lbF, the radius displaced further after the central band of the interosseous ligament was cut, compared with the intact forearm construct (from 1.41 mm intact to 3.00 mm cut; P ≤ .01) (Figs. 3, 4). In pronation, when the loads were increased from the 5-lbF preload to 50 lbF, the radius displaced further when the central band of the interosseous ligament was cut than with the intact forearm construct (from 1.57 mm intact to 2.84 mm cut; P ≤ .01).
      Figure thumbnail gr3
      Figure 3Fluoroscopic images: A baseline intact specimen under 5-lbF preload, and B the same specimen with the central band removed and 50-lbF full load applied.
      Figure thumbnail gr4
      Figure 4Force versus displacement data with representative curve for each of the 6 specimens. A Supination with central band intact. B Supination with central band cut. C Pronation with central band intact. D Pronation with central band cut.

      Discussion

      This study shows that loss of central band integrity produces an increase in ulnar variance despite maintaining all other forearm ligaments. Under 5 lb of axial load, which simulates forces resulting from baseline muscle tone, ulnar variance increased 2 to 3 mm, depending on forearm rotation. Further increase in ulnar variance occurred with progressive loading. This is consistent with previous studies that showed ulnar variance increases during grip.
      • Friedman S.L.
      • Palmer A.K.
      • Short W.H.
      • Levinsohn E.M.
      • Halperin L.S.
      The change in ulnar variance with grip.
      • Tomaino M.M.
      The importance of the pronated grip x-ray view in evaluating ulnar variance.
      Previous studies suggested a correlation between ulnar variance and age. In a cross-sectional study, Bonzar et al
      • Bonzar M.
      • Firrell J.C.
      • Hainer M.
      • Mah E.T.
      • McCabe S.J.
      Kienböck disease and negative ulnar variance.
      reported an association between positive ulnar variance and increasing age. Their study was trying to clarify whether negative ulnar variance was correlated with Kienböck disease. They determined that the correlation was clear in young patients but unclear in older patients. They studied nondiseased wrists as controls and showed an association between positive ulnar variance and increasing age, which obscured the original correlation with ulnar minus wrists in patients with Kienböck disease. Similarly, pathological conditions such as the Essex–Lopresti lesion, in which the central band is injured, result in positive ulnar variance and ulnar impaction syndrome.
      • Darlis N.A.
      • Ferrez I.C.
      • Kaufmann R.W.
      • Sotereanos D.G.
      Step-cut distal ulnar-shortening osteotomy.
      In our model, we showed a relationship between central band excision and positive ulnar variance. This model does not necessarily replicate progressive elongation of the central band of the forearm interosseous ligaments. We believe that clinically, deficiency in the ligamentous restraints of the central band leads to positive ulnar variance, which could be one of several factors contributing to idiopathic ulnar impaction syndrome. We believe that as this ligament stretches and elongates as a result of age and attrition, the forearm progressively shifts into a more varus alignment until a new position of stability is reached as the central band regains tension.
      Because both bones remain parallel and their absolute length does not change, this shift results in an increased relative length of the ulna with respect to the radius at the level of the wrist, therefore inducing positive ulnar wrist variance. As changes occur at the wrist, corresponding changes must also occur at the elbow. We call this the parallelogram effect: the absolute length of the sides remains constant, but as the angles of a parallelogram change, their relative positions (length) change with respect to each other (Fig. 5).
      Figure thumbnail gr5
      Figure 5The parallelogram effect. A normal forearm with healthy central band tension and a normal carrying angle B Central band laxity develops. C the forearm shifts until tissue tension is restored, shifting the forearm into varus and increasing ulnar variance.
      In our model, the humerus was unsupported and allowed to rotate freely. When force was applied, there was a change in the carrying angle and we observed the unsupported elbow rotate into varus, reaching a new position of stability as determined by the remaining intact structures, although this was not measured. Cutting the central band of the interosseous ligament represented the worst-case failure of the central band. Our study demonstrates the parallelogram effect, with changes in ulnar variance resulting from division of the central band.
      This study has limitations inherent in small cadaveric studies (individual anatomical variation, small sample size, etc). This study represented an acute change, and therefore may not be extrapolated to a chronic condition. Repetitive loading may have caused unobserved damage to the remaining soft tissues, causing small attritional damage to accumulate between iterations. In addition, our study measured only axial displacement of the radius relative to the ulna at the wrist; however, the study did not measure motion in any other direction.
      This study demonstrates that the central band has a role in maintaining longitudinal stability and therefore ulnar variance.

      Acknowledgments

      We would like to thank Mr Antonio Tomayo, Ms Sofia Vignolo, and Ms Niurka Nunez for their assistance with lab setup and data collection.

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