Kerrison Sharpness and the Potential of Single-Use Instruments

Kerrisons are one of the most difficult pieces of surgical equipment to clean and sterilize with 100%certainty, and have been identified as the “instrument most likely to cause bioburden/debris events.” (1) In addition to sterilization and the prevention of cross-contamination, published research—as well as the personal experiences of virtually every surgeon—reveal how important it is to use a sharp kerrison rongeur. Having a sharp kerrison cutting blade edge is essential for accuracy, speed, avoiding unplanned tissue damage, and improving the efficacy of associated bone grafts.

An engineering thesis at the University of Manitoba analyzed kerrison cutting face stress, deformation, and degradation. (2). The report contains dozens of images and engineering analysis, including the following illustration of a dull Kerrison cutting edge:

“A surgical plane can be maintained more easily because a sharp elevator [kerrison cutting edge] finds its surgical plane more easily than a dull one.” (3) This is of particular importance in orthopedic and spinal surgery because sharp instruments can be used more safely, and in relevant surgeries result in far fewer unplanned dural repairs. “Sharp bone instruments make operations easier and more accurate.” (3)

This is a critical effect of sharp kerrison cutting blades. Sharp cutting edges “give more precise control.” (3). Dull blades “create micro and macro fractures. When using sharp osteotomes, the bone is cut, rather than fractured.” “bone that is dissected with an elevator [that] is fresh (sharp), the periosteum or periosteal scar has been cleanly stripped off and the bone is raw, ready to accept a graft. Grafts take much better on fresh or refreshed bone than on a scarred bed. This is similar to a skin graft doing poorly on a scarred bed on a leg.” (3).

The current low rate of reported incidents where dull kerrisons caused negative surgical outcomes is in part because surgeons are so accustomed to dull instruments. One study noted that out of 162 cases of dull instruments, 5 re-operations were needed, and the authors identified 1 case of critical harm, 6 cases of moderate harm, and 35cases of low harm to patients. (4). The unnecessarily increased morbidity and added treatment cost—some entirely unreimbursable—related to something as simple as dull instruments is astounding.

The generalized acceptance of dull instruments as a fact of Hospital operations and poor reporting standards, however, led the authors of that study to comment “this report is likely to only be the tip of the iceberg.” (4). Regardless, these were adverse incidents that could have been avoided altogether. Another study of 4,800 surgical instruments revealed issues in more than 15%. (5). Given the number and frequency of surgical instrument use and reprocessing, the total number of errors—largely minor, but potentially severe—requires an actionable solution. 

Kerrisons in particular are infrequently sharpened and poorly tested. Industry standards require kerrison sharpening only 3-4 times per year, or when they have exceeded some set number of uses in surgery. (6) Between these periods, sharpness is normally tested merely by checking if they can effectively cut a paper index card. (6). Initially, ensuring kerrisons are being sharpened on a regular basis requires tracking each kerrison, and maintaining data on when the individual kerrison was last sharpened/repaired and in how many procedures the specific kerrison was actually used to cut bone (not simply where it was available for use). Further, kerrisons become dull faster after each use—the mechanical stress, as shown in Figure2, below, weakens and creates microfractures in the metal causing them to dull at an accelerating rate with use. Effectively addressing this would require dividing kerrisons into cohorts corresponding with prior use such that some cohorts must be sharpened after fewer uses than others. To our knowledge, no hospital currently collects this data. While high-resolution imaging and scanning of instrument blades can provide excellent information, it is virtually never done outside academic & research purposes. (7). Use of an osteotome on a single patient dulls the cutting surface, and a scanning electron microscope study of instrument sharpness showed that after only 15 bites/cuts, 55% of instruments were no longer appropriately sharp. (8). By 45 bites/cuts, that number rose to 89%. (8). For perspective, two established spine surgeons (one Neuro, one Ortho) estimated an average of 50 bites/cuts peroperative spine level for their laminectomies / decompressions—meaning that reusable kerrisons are no longer appropriately sharp after even a single surgery.

While advanced alloys have improved the situation significantly, all cutting blades are faced with two trade offs. First, softer metals are less brittle and prone to fracture, but are far less effective at holding an edge through sterilization, while harder metals can hold an edge through the sterilization process but are highly susceptible to micro-fractures that rapidly dull that edge during surgery, and even suffer breakages of flakes of blade material under stress. Second, a sharper blade (thinner cutting edge) will dull more quickly and is more susceptible to breakage, whereas a duller blade is less prone to breakage. This issue is of significant concern with kerrisons—ideally surgeons want a sharp kerrison that has minimal risk of fracture (meaning a slightly softer alloy that has been sharpened for ideal one-time use sharpness). Hospital economics, however, often drive the purchase of harder, more brittle alloys that require slightly less frequent sharpening but are more prone to stress fractures, making proper cleaning and sterilization ever more difficult.

This is not a merely academic discussion: the FDA has warned that metal surgical instrument fracture is a startlingly common occurrence (often not recognized by the surgical team because the fragments are often very small but still significant), resulting in retained instrument fragments in patients occurring thousands of times per year. (10). These retained fragments lead to a range of serious concerns, including local tissue reaction due to metal allergy, infections, revision surgeries, disability, and even death. (10, citing 11). The FDA additionally warned that MRI imagery (common post spinal fusion) can cause these fragments to migrate and heat, severing internal tissue and causing internal burns—of obviously significant concern in any neurosurgical environment. (11). One study has found that instrument breakage is recorded in only 0.35% of orthopedic surgeries (14), though the actual occurrence is likely far higher due to smaller fragments going undetected and general underreporting. Significantly, only 12% of those fragments noticed by the surgeon were removed, whereas 88%had to be left in situ. (14). The probability of such an occurrence rises dramatically, however, when a kerrison’s cutting edge has been stressed by too much use, has been frequently reprocessed and sterilized, or is made from metal alloys designed to hold a sharp edge through multiple uses. The only realistic solution is the adoption of single-use, sterile, pre-packaged kerrisons made of alloys with a lower “hardness” on the Rockwell Hardness Scale (“HRC” rating)—an alloy that will maintain an exceptionally sharp edge through at least a single surgery without risk of stress fracture. (12); See also, (13). The ability to design a kerrison for a single-use actually provides the unique opportunity to use a metal alloy that optimizes both hardness and sharpness for the specific purpose required.

Lastly, dull kerrisons, as well as pitting and corrosion from repeated sterile processing of kerrisons increases the potential for surgical site infections. One published study has shown an inverse correlation between instrument sharpness and post-sterilization bone contamination—concluding that “disposable osteotomes should be used in surgical procedures.” (15).

To avoid pitting, kerrisons should begin the cleaning process within 15 minutes after use—otherwise they will begin a process of rusting, pitting, and corrosion even before arriving at post-surgery sterile processing, where pitting and corrosion is accelerated. (9). The obvious challenge here is that this is nearly impossible in a surgery that may last many hours. “Pitted instruments can hold microorganisms past sterilization and can lead to surgical site infections.” (16).

The role of kerrisons in spinal surgeries is of particular significance—quickly and accurately removing bone, avoiding damage to soft tissues, and preparing the bone surface to optimize graft acceptance and integration are all facilitated when using sharp kerrisons. 

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References:

(1): https://cheps.engin.umich.edu/wp-content/uploads/sites/118/2019/02/POSTER-AORN-2018-Surgical-Instruments.pdf

(2): https://mspace.lib.umanitoba.ca/server/api/core/bitstreams/30113951-e7a1-464e-899f-7c6af67b231b/content

(3): https://journals.lww.com/plasreconsurg/citation/2005/10001/on_the_maintenance_and_sharpening_of_instruments.10.aspx

(4): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6687421/

(5) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1964625/

(6) https://www.instrumentlearning.com/pub/media/wysiwyg/sharp.pdf; https://www.instrumentlearning.com/pdf/Nov.Dec%202021%20InstrumentWisperer%20Final.pdf

(7) https://www.eventreg.purdue.edu/info/central-service/pdf/cis/210809_July_August-CIS%20Final.pdf

(8) https://pubmed.ncbi.nlm.nih.gov/3897505/

(9) https://www.infectioncontroltoday.com/view/instrument-cleaning-repair-and-handling

(10)https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6687421/

(11)https://wayback.archive-it.org/7993/20170112170154/http://www.fda.gov/Safety/MedWatch/SafetyInformation/SafetyAlertsforHumanMedicalProducts/ucm079030.htm

(12) https://www.sciencedirect.com/science/article/abs/pii/S0195670107000412

(13) https://www.scribd.com/document/30862627/Material-Property-Report

(14) https://pubmed.ncbi.nlm.nih.gov/19047710/

(15)https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0261322

(16)https://www.hpnonline.com/surgical-critical-care/article/21213861/keeping-instruments-patientready

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