Use Your Head: Headless Versus Headed Screws

In recent years, I have noted a shift towards headless screw usage in multiple foot and ankle applications. In my observation, headless screws boast a low-profile design with the ability to bury the screw below the bone’s cortical surface leading to decreased soft tissue adhesion and irritation. But, are we getting the best screw for those perks? 

Over my years in practice, I have used both headless and headed screws. I found that despite that fist bump in the OR, “full hand” compression and impressive “bite” felt in the screwdriver handle while inserting the headless screws; I didn’t observe the correlating level of compression across the osteotomy. This was especially true for fusion sites and I sought after a reason why. After all, this style of screw had a lot of buzz surrounding it, and, what was that full hand tightness on the screwdriver handle with those final rotating turns burying the screw head?  

Upon conducting my research, I found very little recent literature on the topic. However, after a deep dive into the World Wide Web, I was able to glean why I was having such an issue. It seems that with review of various fixation company operative technique guides, most modern generation headless screws, despite pitch angle and threads, etc., are basically and fundamentally a “Herbert” screw.  By definition a Herbert screw is "two sets of threads distally and proximally, separated by a smooth shaft. The leading threads have a greater pitch than the trailing threads; this design allows fragments to be drawn together as trailing threads enter bone. Maximum interfragmentary compression is achieved when trailing threads are advanced completely within bone."¹ The McGlamry text specifically notes that compression of the Herbert screw is far inferior to AO/ASIF headed screws. The McGlamry text goes on to evaluate the Herbert screw's mechanical stability using a study by Shaw stating, "The ASIF 4.0 mm cancellous screw generated a mean maximum compressive load of 17.0 kg compared with 4.4 kg for the Herbert screw in paired cadaveric scaphoid bones. The author noted that the full compressive ability of the Herbert screw could not be realized when trailing threads were not driven through the cortical shell."^1,2  

Throughout my research, I often found recommendations for prefixation clamping when using headless screws.¹ Another study I reviewed was one by Lange and colleagues, utilizing 4.0-mm ASIF cancellous screws and Herbert screws in a cancellous bone model using lumbar vertebrae of calves. This study found that “the pullout strength of the 4.0 mm ASIF screw was 98.4 N. The single and double Herbert screw produced pullout forces of 56.5 N and 129.2 N, respectively. Compressive force generated by the 4.0 mm ASIF screw was 74.5 N and 9.4 N for the Herbert screw. Adding a second Herbert screw increased the compressive load to 27.2 N.”^1,3 

A third study in 2018 found that the “compression force of the Herbert screw was 19.33 +/- 1.0 N.”⁴ This study compared the Herbert screw to other headless designs such as a headless reduction (HLR) screw, which is fully threaded with two thread run outs in the middle of the shaft, and the Mini-Acutrak 2^® (Acumed) screw which is fully threaded without gapping. “The HLR achieved compression force of 47.4 +/- 0.9N. The Mini-Acutrak 2 screw achieved a maximum compression force of 50.98 +/- 1.29 N.”⁴ I was shocked by the difference of force of compression and pullout strength. Converting kg to Newton force units, we see that the previously discussed studies found 17.0 kg of compression in the 4.0 mm ASIF screw which is 166.7 N. The Herbert screw in that study achieved 4.4 kg which converts to 43.1 N of compression load.²

To accommodate for lack of compression by design of the screw, some previous generations of the headless screws that we use today had special driver attachments called, compression sleeves, to achieve compressive forces that were inherently lacking in the screw design itself. “The compression sleeve acts as a conventional lag screw head.  “When the top of the compression sleeve contacts the bone, the fracture gap is closed and compressed as with a lag screw acted as the head, creating a connection between the screw and the screwdriver until the last rotation of the head burying into the proximal fragment, which maintained the compression until the head of the screw fully locked down.”⁵

From what I learned, the main design flaw is that the Herbert screw is that compression does not initiate across the fracture site until the head of the screw is buried into the bone.  This limits compressive forces to the length of the threads of the head of the screw.  The small size of bones in the foot and ankle make this particularly true. In one study, "With over-fastening (of the Herbert screw), the torque increased, but the interfragmentary compression decreased."⁴ On the other hand, "The fully threaded design of the Mini-Acutrak 2 and HLR had no reduction due to over-fasting."⁴ Some osteotomies, such as a central metatarsal shortening osteotomy or oblique akin phalangeal osteotomy, for instance, tend to have a narrow bone shelf as a proximal fragment. In my experience, upon advancing the screw head fully into bone to achieve maximum compression; the screw head threads can sometimes pass across the osteotomy line and into the distal fragment. This can lead to lost compression or even distraction of the fragments without the utilization of prereduction with bone clamps.

Now what about that immense “bite” I felt when putting those headless screws in? One would think that was a sign of great compression. That feedback of great compressive “bite” is explained in McGlamry: "Substantial resistance is met as trailing threads advance through the cortical shell. The perception of significant resistance should not be mistaken for generation of compression across the fracture or impending failure (stripping of the threads).”¹ As Lin et al found that, “… the Herbert screw demonstated increased torque with over-fastening but decreased interfragmentary compression.”⁴ Torque is defined as, “a measure of the effectiveness of such a force that consists of the product of the force and the perpendicular distance from the line of action of the force to the axis of rotation.”⁶

Headless screws are not without their perks, however. Reduction of soft tissue irritation and intra-articular fixation are main benefits. In my experience, use of the fully threaded-type headless screws are ideal for hammertoe fusions. I prefer the fully threaded screw in hammertoe surgery, due to the ability to allow the patient to get their foot wet after incision healing compared to six weeks with K-wires. Also, the fully threaded screw does not have a runout like a Herbert-type screw that often has its runout near a joint, which can pose a problem since the “runout is the weakest point of a partially threaded screw.”¹

Utilizing mechanical research of compression, pull out strength, and personal experience, I believe that headed screws offer an ideal level of compression across joints, such as the subtalar joint, tarsometatarsal joints, first metatarsophalangeal joint, ankle joint and obviously for fracture treatment of the foot and ankle. In fact, in some of these instances I would even add a washer to increase compression across the fusion/fracture site. Surgical fixation definitely comes down to surgeon preference and experience with certain fixation materials/constructs. With knowledge of the limitations and benefits of those materials we can become better surgeons, with better patient satisfaction and outcomes.

Douglas Appel, DPM, FACFAS is a dual board-certified Foot and Ankle surgeon in private practice in Wilmington, Delaware.

References
1. Ray R. Methods of Osseous Fixation. In: McGlamry's Comprehensive Textbook of Foot and Ankle Surgery. (3rd ed.) Lippincott Williams & Wilkins;2001:65–106.

2. Shaw JA.  A biomechanical comparison of scaphoid screws.  J Hand Surg [Am]. 1978;12:347-353.

3. Lange RH, Vanderby Jr R, Engber WD, Glad RW, Purnell ML. Biomechanical and histological evaluation of the herbert screw. J Orthop Trauma. 1990;4(3):275-282.

4. Lin CC, Lin KP, Huang CC, et al. Partially threaded headless screw may benefit adequate interfragmentary compression and reduced driving torque for small bone fixation. J Orthop Surg (Hong Kong). 2018;26(1):2309499018760130.

5. Technique Guide. 2.4mm and 3.0mm Headless compression screws. Synthes 2006. Available at: https://synthes.vo.llnwd.net/o16/LLNWMB8/US%20Mobile/Synthes%20North%20America/Product%20Support%20Materials/Technique%20Guides/DSUSTRM09161087_2-4_3mmHeadlessCompScr_TG_150dpi.pdf . Accessed July 9, 2021.

6. Torque. Merriam-Webster.com Dictionary. Available at: https://www.merriam-webster.com/dictionary/torque. Accessed June 30, 2021.