Dimensional radiographic analysis of distal interphalangeal joint morphometry for fusion implant suitability – A Singapore population study

Abstract

Background

Distal interphalangeal joint (DIPJ) fusion is a commonly used surgical technique for the treatment of osteoarthritis. It can be challenging due to the small size of the phalanges.

Objectives

This study aims to determine the nominal dimensions of middle and distal phalanges of digits in the Singapore population via radiographic measurements, to facilitate the selection of appropriate implants for DIPJ fusion.

Materials and methods

De-identified plain radiographs of 250 fingers from 250 adult Singaporeans from July 2020-2023 were retrospectively obtained and measured using the authors’ institution’s radiograph software. Lateral and anteroposterior diameters of the distal and middle phalanges were recorded and compared with the smallest commercially available headless compression screw (Medartis 1.7 mm) currently available in Singapore.

Results

Female phalanges were smaller than that of males. Anteroposterior diameters of middle phalanges in females were significantly smaller than that of males in all fingers except for the thumb and middle finger (p < 0.05*). Anteroposterior diameters of distal phalanges in females were significantly smaller than that of males for all fingers (p < 0.05*). The compatibility rate of the Medartis 1.7 mm cannulated screw between the two genders was found to be on average: 94% in the thumb, 44% for the index finger, 38% for the middle finger, 30% for the ring finger and 12% for the little finger.

Conclusion

Apart from the thumb, less than 50% of the digits in the Singapore population were found to be compatible with the smallest available screw. The findings highlight the need for more suitable fixation devices or techniques for DIPJ fusion.

Introduction

Distal joint fusion is of great clinical applicability due to the rapidly aging population in Singapore and in many first world economies. Osteoarthritis of the hands is on the rise and this common degenerative condition frequently involves finger DIPJs and thumb IPJs. Specifically, at least 60% of individuals aged 60 years and above would have distal interphalangeal (DIP) joint arthritis. ¹ DIPJ arthroplasty has been reported as another treatment option, however it has an overall complication rate of 15% and a re-operation rate of 8%, and failures are salvaged by DIPJ arthrodesis. As such, DIPJ arthrodesis is still a common treatment for osteoarthritis. ²

The aim of finger distal interphalangeal joint (DIPJ) and thumb interphalangeal joint (IPJ) arthrodesis is to produce a stable, painless joint with minimal irritation from the hardware used. It is a treatment modality for degenerative, inflammatory, traumatic, or infective causes with significant joint involvement resulting in pain, deformity and instability.^3,4

The choice of surgical approach and implants used are important factors that affect the treatment outcomes. However, this remains challenging due to the small size of the phalanges and limited zone of surgery. Commonly used surgical methods for this procedure include Kirschner wires, cerclage wires and screws; each have their advantages and disadvantages, and hence no single technique has gained universal consensus. ⁵

The use of headless compression screws for DIPJ and IPJ fusion has gained traction as it allows for greater compression across the joint due to the variable pitch of the screw threads and convenience to the patient with no need for hardware removal. ⁶ The bending strength is also increased as compared to other methods like tension band wiring. ⁷

To minimise the risk of complications of screw thread penetration such as injury to the germinal or sterile matrix or implant fractures across the joint, it is imperative that the screw fits into both the distal and middle phalanges. ⁷ This is technically difficult due to the small sizes of the distal and middle phalanx, which is even smaller in Asian populations, as compared to Caucasian populations. Although a study has been done to determine the nominal dimensions of fingers in a South Korean population, ⁵ no similar study was performed in the Southeast Asia region.

This study aims to determine the nominal dimensions of our patients’ middle and distal phalanges of all five digits in Singapore via radiographic measurements. This will be compared to data from a South Korean population and analysed in comparison with the smallest available screw implant in Singapore.

Materials and methods

This is a retrospective cross-sectional study that was reviewed and approved by the SingHealth Centralised Institutional Review Board (CIRB) Ref: 2022/2280 on 19 June 2022. No patient consent was required as per CIRB guidelines as de-identified data was used. STROBE guidelines were used for reporting of data.

Standard anteroposterior and lateral normal radiographs of 250 fingers were obtained from 250 adult Singaporean patients who presented to the authors’ hand surgery clinics for finger conditions. Radiographs were performed as part of standard treatment to rule out bony involvement. The inclusion criteria included patients 21 years of age and older. Patients who had prior finger injuries such as fractures or other underlying bone or joint conditions such as arthritis were excluded from our study. Rotated radiographs were also excluded. Other patient demographics such as gender was also recorded.

The authors’ institution’s digital radiograph software (Carestream by Phillips) was used to measure the diameters of the phalanges. The radiographs were calibrated for measurement on a digital viewer. The diameters of the middle and distal phalanges were measured on both images as follows: The middle phalanx was measured as the narrowest diameter of the medullary canal (Figure 1(a)), while the distal phalanx was measured as the narrowest diameter of the cortical bone (Figure 1(b)), which is a similar method used by Song et al. 2012 in their South Korean population study. ⁵

Figure 1.

(a).White arrows showing method of measuring lateral and AP diameter of middle phalanx using narrowest diameter of medullary canal; (b). White arrows showing method of measuring lateral and AP diameter of distal phalanx using narrowest diameter of cortical bone.

These measurements were then compared with the smallest commercially available headless compression screw (Medartis 1.7 mm) used in Singapore for arthrodesis of the distal interphalangeal joint. The screw was considered to fit if the clearance between the leading thread (1.7 mm) and the measured diameter of the phalanx was greater than 0.2 mm, and oversized if they exceeded this on either lateral or anteroposterior radiograph. 0.2 mm was chosen as the smallest width between the cortex and the screw required to minimise the risk of the cortical breach; this allows for 0.1 mm clearance on each side of the screw. The compatibility rate was calculated as the percentage of digits that allowed the screw to fit, based on the 250 individual measurements. This data was also compared to the data collected by the South Korean study ⁵ to show the difference in phalangeal sizes between different Asian populations.

All statistical analyses were performed using SPSS software (IBM Corp, Chicago, IL, USA). The student t-test was utilised to compute the statistical significance of differences between groups, with p-value of less than 0.05 considered to be significant.

Results

Patient demographics are summarised in Table 1. Finger dimensions from two hundred and 50 patients (84 females, 166 males) were collected. These measurements are in Table 2 and Table 3. The AP diameter of middle phalanges in females was significantly smaller than that of males for all fingers except for the thumb and middle finger (p < 0.05*). The AP diameter of distal phalanges in females was significantly smaller than that of males for all fingers (p < 0.05*).

Table 1.

Patient demographics.

Patient characteristics
	Thumb	Index	Middle	Ring	Little
Gender – no.
Males	27	37	31	37	34
Females	23	13	19	13	16
Age - year
Mean	48.8 ± 17.9	41.9 ± 16.1	44.0 ± 17.4	41.1 ± 14.9	42.4 ± 17.1
Range	18 – 89	18 – 76	19 – 78	20 - 72	17 - 76

Table 2.

Finger dimensions of the Singapore population.

Gender	Middle phalanx		Distal phalanx
	Lateral diameter/mm	AP diameter/mm	Lateral diameter/mm	AP diameter/mm
Thumb
Female	2.70 ± 0.72	4.81 ± 0.94	3.31 ± 0.60	6.60 ± 0.84
Male	2.97 ± 0.65	5.28 ± 0.97	3.62 ± 0.77	7.38 ± 1.13
p value	0.161	0.091	0.126	0.01 a
Index finger
Female	1.69 ± 0.39	3.18 ± 0.83	2.74 ± 0.33	3.98 ± 0.34
Male	1.95 ± 0.50	3.75 ± 0.72	3.05 ± 0.78	4.60 ± 0.56
p Value	0.088	0.05 a	0.174	0.00 a
Middle finger
Female	1.65 ± 0.44	3.83 ± 0.67	3.04 ± 0.33	4.54 ± 0.41
Male	1.83 ± 0.42	3.89 ± 0.78	3.26 ± 0.45	4.90 ± 0.58
p Value	0.151	0.768	0.074	0.05 a
Ring finger
Female	1.56 ± 0.66	3.12 ± 0.74	2.84 ± 0.43	4.17 ± 0.40
Male	1.72 ± 0.70	3.70 ± 0.82	3.01 ± 0.71	4.68 ± 0.52
p Value	0.462	0.05 a	0.419	0.005 a
Little finger
Female	1.45 ± 0.63	2.77 ± 0.85	2.32 ± 0.41	3.02 ± 0.38
Male	1.51 ± 0.29	3.32 ± 0.71	2.49 ± 0.54	3.66 ± 0.72
p Value	0.661	0.05 a	0.245	0.001 a

^aLegend: AP = Antero-Posterior.

Data represent mean (in millimetres) ± SD.

Table 3.

Comparison of digit radiographic measurements between the Singapore and Korean population.

		Middle phalanx		Distal phalanx
		Lateral diameter/mm	AP diameter/mm	Lateral diameter/mm	AP diameter/mm
Thumb	SG	2.85 ± 0.69	5.06 ± 0.98	3.48 ± 0.70	7.02 ± 1.07
	KR	4.17 ± 0.68	5.04 ± 0.64	5.04 ± 0.73	6.39 ± 0.76
	p value	0.001*	0.904	0.001*	0.001*
Index	SG	1.88 ± 0.49	3.60 ± 0.78	2.97 ± 0.70	4.44 ± 0.58
	KR	2.09 ± 0.56	3.68 ± 0.57	3.13 ± 0.52	4.40 ± 0.52
	p value	0.05*	0.560	0.198	0.717
Middle	SG	1.77 ± 0.43	3.87 ± 0.74	3.18 ± 0.42	4.76 ±0.54
	KR	2.10 ± 0.55	3.82 ± 0.61	3.17 ± 0.55	4.74 ± 0.63
	p value	0.001*	0.713	0.459	0.865
Ring	SG	1.68 ± 0.68	3.55 ± 0.83	2.97 ± 0.65	4.55 ± 0.54
	KR	1.95 ± 0.53	3.71 ± 0.50	3.05 ± 0.49	4.61 ± 0.56
	p value	0.05*	0.246	0.489	0.587
Little	SG	1.49 ± 0.42	3.14 ± 0.79	2.44 ± 0.50	3.45 ± 0.69
	KR	1.83 ± 0.50	2.75 ± 0.55	2.64 ± 0.51	3.54 ± 0.55
	p value	0.00*	0.005*	0.050	0.472

Legend: SG = Singapore, KR = South Korea.

Data represent mean (in millimetres) ± SD.

Significant p values have been bolded and labelled with an asterisk *

In comparison to the Medartis 1.7 mm cannulated screw, the compatibility rate was on average 94% (96% male, 91% female) in the thumb, but 44% (51% male, 23% female) for the index finger, 38% (45% male, 26% female) for the middle finger, 30% (32% male, 23% female) for the ring finger and 12% (9% male, 19% female) for the little finger.

A comparative analysis was performed with the data from the South Korean population study, and it was found that the mean lateral diameter of the middle phalanx for all five digits (thumb p < 0.001*, index finger p < 0.05*, middle finger p < 0.001*, ring finger p < 0.05*, little finger p < 0.001*) and the mean lateral diameter of the distal phalanx of the thumb (p < 0.001*) in the Singapore population was significantly smaller the Korean population.

In contrast, the mean AP diameter of the middle phalanx of the little finger (p < 0.01*) and the mean AP diameter of the thumb distal phalanx (p < 0.001*) in the Singapore population was significantly larger than the Korean population. There are no statistically significant differences found for the dimensions of the other phalanges when compared to the Korean population. All these results are demonstrated in Table 1.

Discussion

In available literature, non-union rates for small joint arthrodesis in the hand range widely from 0 to 30%.^8–11

A study by Engel et al. in 1977 ¹² looked specifically at DIPJ arthrodesis, and in their case series of 30 patients, the non-union rates between screw and K wire arthrodesis was identical but time to union and return to work was faster in the screw arthrodesis group. Ideally, fixation should be performed with screw arthrodesis for faster recovery, but this may not always be compatible with the size of the finger based on our radiographic measurements. It may thus be beneficial to develop more suitable screw sizes for the Singaporean population.

There are a multitude of factors that contribute to bone union. Besides surgical technique, age, comorbidities, patient compliance and even cigarette smoking are all important attributes but can be difficult to sub-analyse. As surgeons, the main variable we can control is our surgical technique, which includes selection of appropriate implants with good bone to bone contact. DIPJ or IPJ arthrodesis using a headless compression screw or K wires are effective surgical treatments for a plethora of distal joint pathologies, however due diligence to the size of the middle or distal phalanx must be considered for suitable implant selection.

As seen in both our paper and the Korean skeletal study paper, amongst the various diameters measured, the smallest diameter measured is the middle phalanx lateral view. As seen in the ring and little fingers, the size of the medullary canal of the middle phalanx (or proximal phalanx of the thumb) is too small (<1.7 mm) for the screw to be inserted, and may cause cortical breakage during screw insertion. Hence, the rate limiting size may be the lateral diameter of the middle phalanx.

The ideal size of the screw needs to take into these two factors; For the distal phalanx, the screw needs to be smaller than the smallest diameter of both the lateral and AP views to minimise risk of both cortical and nailbed damage. However, for the middle phalanx, the screw needs to be large enough to achieve sufficient purchase and a stronger fixation. Our data and the data from the Korean study consistently demonstrated that the smallest diameter was the middle phalanx lateral view. This suggests that the ideal screw size should be at least 0.2 mm smaller than the middle phalanx lateral diameter.

There are many headless compression screws in the global market. The Medartis 1.7 mm screw was selected as a comparison for the authors’ radiographic measurements as it is currently the smallest one available in Singapore. It is also smaller than the screw used in the Korean study, ⁵ in which clinical results of DIPJ arthrodesis using an Acutrak 2.0 mm fusion screw were reported.

The limitations of this study are that this is a retrospective review, and the Singapore population is a heterogenous one, which comprises of four main racial groups: the Chinese, Indians, Malays, and Eurasians. This may lead to variability in data collection as it is not as homogenous as that of South Korea. Moreover, our results may not be an accurate representation for arthritic digits as the joints may exhibit pathology such as deformity, osteophytes, subchondral cyst formation and poor bone stock.

In the horizon, the invention of smaller screws and perhaps even DIPJ arthroplasty implants can be developed to suit the rising functional needs of an aging yet independent population. Further research into more novel techniques of surgical fixation should also be undertaken.