Ulnar shaft diameter as it relates to plate osteosynthesis: A cadaveric study

Abstract

Background

Ulnar shaft fractures are common and the standard of care treatment is with 3.5 ​mm plating. The purpose of this study was to measure diameters along the length of the ulna to provide information on screw length and appropriateness of 3.5 ​mm screws and plate location.

Methods

Ten embalmed cadaveric ulnas were dissected free of all soft tissues. The length of the ulna was measured and then divided by four. Transverse osteotomies were performed along the length of the ulna at ¼, ½ and ¾ its full length. The diameter of the ulna at these locations was then measured in a radio-ulnar direction as well as a volar-dorsal direction.

Results

There were five female and five male specimens with an average age of 83.6 and 78.8 respectively. The smallest diameter in both the female and male specimens was in the distal ¾ shaft measured in a volar-dorsal direction with an average of 9.1 ​mm and 12 ​mm. The average volar-dorsal measurement was less than radio-ulnar measurement at all locations. The diameters increased along the ulna heading proximally.

Conclusion

This study provides information pertinent to screw width and length with respect to the ulnar shaft and calls in to question the use of 3.5 ​mm screws in the distal and midshaft ulnar diaphysis in females. Removal of 3.5 ​mm screws in the mid and distal ulnar shaft in females can leave a hole corresponding to 23% and 27% of the bone which can be concerning for a stress riser.

1. Introduction

Fractures of the ulnar shaft are a common injury that can be seen both in isolation and associated with radial shaft fractures. The mechanism of injury is typically a direct blow to the forearm with the majority of ulnar shaft fractures located in the distal third.1, 2, 3, 4 Various treatment options for ulnar shaft fractures have been described including early mobilization,¹ plaster splint immobilization,³ functional bracing,⁴ intramedullary fixation² and open reduction internal fixation.^2,5, 6, 7, 8 Generally, nondisplaced fractures may be treated by nonoperative means and unstable fractures with open reduction internal fixation. Instability in isolated ulnar shaft fractures warranting operative fixation has been defined as >50 ​% displacement, 10 degrees of angulation, proximal 1/3 shaft fractures with an unstable proximal radioulnar joint or fractures associated with instability of the distal radioulnar joint. Displacement and angulation of the ulnar shaft can significantly impair the interosseous membrane and distal radioulnar joint leading to impaired forearm function.

The current standard of care for operative fixation is open reduction internal fixation with a 3.5 ​mm limited-contact dynamic compression (LCDC) plate along either the subcutaneous, dorsal or volar border with six cortices on either side of the fracture.5, 6, 7^,9 Anecdotally, in distal third ulnar shaft fractures, 3.5 ​mm plating with 3.5 ​mm screw fixation can seem overly large for the bone. Prominent ulnar hardware with irritation prompting removal is not uncommon. Teunissen reviewed a cohort of patients who underwent ulnar shortening osteotomy with plate fixation and 47 ​% had complaints of hardware irritation with 32 ​% ultimately requiring hardware removal.¹⁰ Removal of forearm plates has been shown to increase the rate of refracture.¹¹ The clinical concern is that for narrow ulnas, placing and later removing symptomatic 3.5 ​mm plates and screws can create considerable stress risers leading to potential fracture or refracture.

The aim of this study is to determine if 3.5 ​mm screws and plates are appropriate for the diameter of the ulna and to provide general length measurements of screws along the shaft of the ulna.

2. Methods

Ten embalmed cadaveric specimens were obtained. The ulna was completely dissected free of all soft tissues (Fig. 1). Utilizing an EZ-Cal IP54 digital caliper (iGaging, San Clemente, California) calibrated to 0.025 ​mm, the length of the ulna was measured from the tip of the olecranon to the tip of the ulnar styloid. The length was then divided by four to assess diameters of the ulna at ¼, ½ and ¾ of its full length along the diaphysis. These measurements were then marked on each specimen with a marking pen along the subcutaneous border. Utilizing an oscillating saw, a transverse osteotomy was made at each mark (Fig. 2). The diameter of the ulna at each osteotomy was measured twice at the widest two points passing through the medullary canal in a radio-ulnar direction and volar-dorsal direction. The radio-ulnar diameter measurements would therefore be an apt measurement of screw lengths passing straight through the medullary canal for a plate placed along the subcutaneous border and the volar-dorsal diameter measurements would be an apt measurement for screw lengths for plate placement either dorsally or volarly (Fig. 3). Amongst all male and female specimens, averages and standard deviations were calculated.

Fig. 1.

Ulna stripped of soft tissues with subcutaneous border marked.

Fig. 2.

Osteotomies completed at 1/4th, ½ and 3/4th total length of the ulna. Markings along subcutaneous border at the various measurements can be seen at osteotomy sites.

Fig. 3.

Schematic of measurements at distal 3/4th osteotomy site.

3. Results

Five female and five male specimens were dissected. There were three right and two left ulnas in the male group and three left and two right ulnas in the female group. The ages of the specimens ranged from 57 to 90 years old with the average male age of 78.8 and average female age of 83.6. The average length of the ulna amongst male specimens was 266.8 ​mm and 244.9 ​mm amongst female specimens. Table 1 illustrates the measurements in millimeters of the diameters along the ulna shaft at one fourth, half and three fourths of each ulna's full length. The average male diameter in the volar-dorsal direction was 16.88 ​mm, 13.92 ​mm and 12.01 ​mm at the proximal 1/4th, ½ and distal 3/4th respectively. The average male diameter in the radial-ulnar direction was 19.06 ​mm, 16.52 ​mm and 13.73 ​mm at the proximal 1/4th, ½ and distal 3/4th respectively. The average female diameter in the volar-dorsal direction was 13.16 ​mm, 11.06 ​mm and 9.17 ​mm at the proximal 1/4th, ½ and distal 3/4th respectively. The average female diameter in the radial-ulnar direction was 15.71 ​mm, 14.05 ​mm and 11.36 ​mm at the proximal 1/4th, ½ and distal 3/4th respectively. The average volar-dorsal measurements were less than the average radio-ulnar measurements at all points along the ulna in both males and females. The smallest average diameter values were found in the distal ¾ shaft measuring from volar to dorsal in both the male and female specimens. The values increased heading proximally along the ulnar shaft.

Table 1.

Ulnar shaft diameters.

	Age	Laterality	Ulna Length	Proximal 1/4 Diameters		Midshaft 1/2 Diameters		Distal 3/4 Diameters
				Radial-ulnar	Volar-dorsal	Radial-ulnar	Volar-dorsal	Radial-ulnar	Volar-dorsal
Male
1	81	R	253.14	16.29	16.9	10.34	16.8	12.72	10.78
2	57	L	267.64	18.95	15.13	18.28	10.57	13.78	11.9
3	90	R	272.62	18.2	21.93	21.37	11.93	14.2	13.01
4	80	R	266.04	18.49	14.25	12.9	17.51	12.98	10.9
5	86	L	274.79	23.38	16.17	19.7	12.8	14.97	13.45
Average	78.8		266.85	19.06	16.88	16.52	13.92	13.73	12.01
Std Dev			7.56	2.34	2.68	4.20	2.74	0.82	1.08
Female
1	87	L	248.15	13.39	14.47	9.56	15.05	11.27	8.48
2	80	L	246.2	14.58	11.45	13.61	9.82	10.39	8.46
3	79	L	245.65	14.61	13.58	16.08	9.72	11.65	9.5
4	84	R	241.29	16.19	12.8	14.54	10.56	11.35	9.75
5	88	R	243.2	19.76	13.51	16.44	10.17	12.15	9.67
Average	83.6		244.90	15.71	13.16	14.05	11.06	11.36	9.17
Std Dev			2.40	2.21	1.01	2.47	2.01	0.58	0.58

4. Discussion

Ulnar shaft fractures are common injuries. Open reduction internal fixation with 3.5 ​mm plating remains standard of care for operative treatment of ulnar diaphyseal fractures. It has been our experience that, intraoperatively, the 3.5 ​mm plate and screws can appear overly large compared to the diameter of the bone. We chose to measure the diameters of the ulnas at ¼, ½ and ¾ of the length of the ulna to have representative measurements of areas that would commonly be referred to as proximal third diaphyseal, midshaft and distal third diaphyseal. There have been a few studies evaluating intramedullary diameter of the distal ulna in relation to DRUJ prosthesis,^12,13 intramedullary implants,¹⁴ osteotomies of proximal ulna¹⁵ as well as anthropomorphic studies observing sex differences in the length of the ulna and midshaft diameter for sex identification,¹⁶ however no studies to the authors knowledge measuring diameters of the ulna along its length nor in relation to plate osteosynthesis.

Limitations of this study include a small sample size using elderly cadaveric specimens, which may not be representative of a younger population. Elderly bone is known to undergo expansion and an increase in cross-sectional area in response to the weakening material properties of bone which may lead to increased diameter sizes in this cohort.¹⁷ The digital caliper (EZ-Cal IP54 Digital Caliper iGaging, San Clemente California) used is only calibrated to 0.025 ​mm limiting the precision of measurements.

In this anatomic study we showed that the volar-dorsal diameter of the ulna is smallest in the distal third in both the male and female specimens at 12 ​mm and 9.7 ​mm respectively. The distal third of the ulna is where the majority of fractures occur.1, 2, 3, 4 Interestingly 3.5 ​mm plates have a width of 10–11 ​mm depending on whether stainless steel or titanium (DePuy Synthes) making them wider than the average female volar-dorsal diameter in the distal third. It is therefore reasonable to suggest in females that in the distal third of the ulna, 3.5 ​mm plating would fit better if placed on the dorsal or volar surfaces rather than along the subcutaneous border where the plate would be wider than the bone itself. This also does not take into consideration the tubular nature of the ulna compared to a plate with a larger radius of curvature which could mean even larger diameters of the ulna may still not fit appropriately with the plate.

A general idea regarding screw lengths during plate osteosynthesis anywhere in the body is important. Quite often a mismeasured screw length in the operating room can be tempered by prior knowledge of what an appropriate length should be. This study provides information on screw lengths along various points of the ulna for both subcutaneous and volar or dorsal plate placement.

Removal of symptomatic forearm plates, which is not uncommon,^10,18 has been a known risk factor for refracture by leaving behind multiple holes in the bone.¹⁹ This has been shown to occur after removal of both large fragment plates and small fragment plates.²⁰ Over the past 50 years, the recommendation to fix forearm fractures with 4.5 ​mm plates has moved to small fragment or 3.5 ​mm plates.⁹ A 2.5 ​mm drill bit is necessary for insertion of a 3.5 ​mm screw. If the plate is positioned perfectly central along the subcutaneous border in the distal third this puts a hole of around 27% of the diameter of the ulna in the average female and 21% in the average male and if positioned centrally on the volar or dorsal border approximately 22% in the average female and 18% in the average male. In addition, at the midshaft level, a subcutaneous plate placement with use of a 2.5 ​mm drill bit would place a hole of 23% of the cortical diameter in the average female and 18% in the average male. McBroom showed that in canine femora a drill hole diameter to bone diameter ratio of 20% reduced the bending strength of the bone to 62% of its initial strength.²¹ This can become more problematic if the plate is placed eccentrically on the bone or the drill bit angle creates a hole such that a unicortical hole is drilled through all cortex. A diaphyseal defect of 50% of the cortex can lead to strength reductions of 60–90%.²² The above stated percentages do not consider the screw threads which create and leave a larger hole when removed than just the core diameter of the screw alone.

Most 3.5 ​mm plates today have the option to insert 2.7 ​mm screws using a 2.0 ​mm drill bit instead of 3.5 ​mm screws. To our knowledge there have been no biomechanical studies evaluating 3.5 ​mm plates with the use of 2.7 ​mm screws compared to 3.5 ​mm plates with 3.5 ​mm screws. However there has been a recent biomechanical study comparing 2.7 ​mm and 3.5 ​mm locking compression plates for midshaft ulna fractures.²³ They showed that both 3.5 ​mm and 2.7 ​mm plates maintained stability of the fracture throughout nondestructive testing without plastic deformation although 3.5 ​mm plates limited interfragmentary motion significantly more than the 2.7 ​mm plate. However, there was no difference between ultimate load to failure between the two plates and that under destructive loading the 3.5 ​mm plates failed more abruptly with refracture at the distal screw hole whereas the 2.7 ​mm plates plastically deformed. They concluded that because of the increased flexibility of the 2.7 ​mm plate, this may decrease stress shielding and decrease incidence of refracture. Overall, the authors felt there were biomechanical benefits of the 2.7 ​mm plate as opposed to the 3.5 ​mm plates. In addition, ulnar shortening systems recently have been using 2.7 ​mm plating systems as opposed to traditional 3.5 ​mm plating. Jungwirth-Weinberger compared 2.7 ​mm–3.5 ​mm plating for ulnar shortening osteotomies and showed a faster time to bony consolidation with 2.7 ​mm plating compared to 3.5 ​mm plating with no difference in complication or implant removal rates.²⁴ In summary, 2.7 ​mm screws placed into a 3.5 ​mm plate or the use of 2.7 ​mm plates may be more appropriate options for the treatment of ulnar shaft fractures; however further clinical studies need to be undertaken.

In conclusion, this study provides information regarding average screw lengths at various locations along the ulnar shaft with either subcutaneous and dorsal or volar plate placement. It also raises concerns regarding use of 3.5 ​mm screws at certain locations along the shaft of the ulna, namely in the distal third. Finally, it also questions 3.5 ​mm plate placement along the subcutaneous border in the distal third and midshaft in female patients given the small diameter of the bone as compared to the width of a 3.5 ​mm plate.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.