Evaluation of surgical duration using two different burrs for percutaneous endoscopic mini-hemilaminectomy in cats

Abstract

Objectives

This study aimed to compare the influence of two burr types on surgical time during percutaneous endoscopic mini-hemilaminectomy (PE-MHL) in feline cadavers.

Methods

A total of 12 skeletally mature domestic shorthair cat cadavers underwent bilateral PE-MHL at L4–L5 using either a diamond or a cutting burr in alternating order. Surgical duration, surgical side, postoperative CT bone window area and the percentage of lamina removed were compared. Approximately 30% laminar removal was targeted. Data were expressed as mean ± SEM and analysed using Shapiro–Wilk and Levene’s tests, followed by Student’s t-tests (P ⩽0.05).

Results

All procedures were completed successfully. Mean procedure time was significantly shorter with the diamond burr (20.85 ± 1.44 mins) than with the cutting burr (25.62 ± 1.49 mins; P = 0.03). The surgical side had no significant effect on any parameter. Mean bone window areas were comparable between burrs (68.42 ± 2.02 mm² vs 63.92 ± 3.19 mm²; P = 0.27). The overall mean of lamina removed was 32.38 ± 2.31%, closely matching the intraoperative target. Three cutting burrs broke, whereas no failures occurred with diamond burrs.

Conclusions and relevance

Both burr types were effective for PE-MHL in feline cadavers. The diamond burr enabled more efficient, precise and controlled bone removal, resulting in smoother bone margins and consequently reduced procedure times. These findings suggest that the diamond burr may be a more reliable instrument for controlled bone removal under endoscopic conditions and provide the basis for future in vivo studies.

Introduction

Intervertebral disc herniation (IVDH) in cats has traditionally been regarded as rare, although a recent study reported a higher incidence than previously described, possibly reflecting improved recognition and increased availability of advanced diagnostic imaging. ¹ Among the various forms of IVDH, intervertebral disc extrusion (IVDE) is most frequently encountered, with a clear predominance in the lumbar region of the feline spine.^1,2 For thoracolumbar IVDH, including IVDE, intervertebral disc protrusion (IVDP) and mixed forms, conservative and surgical treatment may yield comparable outcomes. ¹ However, cats managed surgically typically present with more severe neurological deficits on admission, which may reflect selection bias between treatment groups. ¹ Several open surgical techniques have been described for spinal cord decompression in cats, including hemilaminectomy,^{3 –7} laminectomy ⁸ and foraminotomy. ⁹ In recent years, minimally invasive spine surgery (MISS) has gained increasing attention, particularly in dogs,^{10 –19} with only a single report describing its use in a cat. ²⁰

A recent cadaveric study comparing percutaneous endoscopic mini-hemilaminectomy (PE-MHL) with standard open mini-hemilaminectomy (SO-MHL) in cats demonstrated the feasibility of the endoscopic approach and its potential to shorten procedure time. ²¹

In this study, surgical duration of PE-MHL largely depended on bone exposure time. ²¹ According to manufacturer recommendations, a diamond burr was used for PE-MHL, whereas SO-MHL was performed with a cutting drill bit to reflect clinical conditions, thereby precluding direct comparison between the two techniques. ²¹ Diamond burrs are indicated for precise bone removal in confined anatomical spaces near delicate structures, where controlled drilling and smooth bone margins are essential.^22,23 Their use, however, generally increases procedure time.^22,23 In contrast, cutting burrs, more commonly used for open mini-hemilaminectomy in dogs and cats, allow rapid removal of cortical and cancellous bone,^22,23 but require careful handling near neural or vascular structures because they lack protective features and may cause soft tissue injury upon contact.

Based on previous findings showing that PE-MHL duration depends largely on bone exposure time ²¹ and the lack of comparative data on burr performance in small animal neurosurgery, the present study aimed to refine the technique further. Specifically, the objective was to evaluate the influence of burr type on surgical duration by directly comparing diamond and cutting burrs during PE-MHL in feline cadavers. The hypothesis was that the use of cutting burrs would reduce procedure time during PE-MHL.

Materials and methods

Set-up and planning

The cadavers of 12 domestic shorthair (DSH) cats were collected for this study. The use of the cadavers was approved by the Ethics Committee of the Faculty of Veterinary Medicine of the LMU Munich (approval number 462-06-06-25). Cadavers were obtained from client-owned cats that had died or been euthanased for reasons unrelated to this investigation, with verbal owner consent obtained for research purposes in all cases. According to national regulations, owner consent was not required for the use of cadavers in research. Inclusion criteria comprised breed (DSH), skeletal maturity and the absence of pre-existing spinal pathology. Orthogonal radiographs (Luminos dRF Max; Siemens) of the thoracolumbar spine were taken to rule out obvious osseous abnormalities or congenital malformations. All radiographs were evaluated by a board-certified surgeon (MK), and in the absence of detectable pathologies, the cadavers were stored at –20°C and thawed to room temperature for 24 h before the procedures. CT of the thoracolumbar spine was performed to further exclude bony pathologies more precisely and to enable comparison of postoperative measurements.

For surgical preparation, hair over the thoracolumbar region was removed with clippers to expose the surgical site. The cats were positioned in sternal recumbency, secured to the operating table and surgically draped to simulate realistic conditions. All procedures were performed by the same board-certified surgeon (MK) within a 1-week period. Concurring to published reports, the lumbar spine in cats is disproportionately affected by IVDE.^2,5,6,8 Furthermore, morphometric data indicate that the feline lumbar spine, particularly the L4–L5 region, represents a clinically relevant area. ²⁴ Because of its consistent anatomical features and reproducible spatial conditions, this segment also provides a reliable basis for experimental comparison. Therefore, the L4–L5 segment was selected as the target site for this pilot study.

Surgical procedure

PE-MHL was performed using the SpineTIP system (interlaminar approach; Karl Storz), with the endoscopic set-up previously described in a pilot study. ²¹ This MISS system provides suction burrs equipped with cutting drill bits enclosed within a protecting sheath (Figure 1b) and diamond burrs with distally unsheathed, round drill bits (Figure 1a). In accordance with the modification established in the pilot study and to enable direct comparison, the diamond burr was combined with the protecting sheath of the cutting burr (Figure 1c).

Figure 1

Burrs of the SpineTIP system (Karl Storz) and the modification used for percutaneous endoscopic mini-hemilaminectomy (PE-MHL) in cats. (a) Diamond burr; (b) cutting burr with integrated protective sheath as provided by the manufacturer; (c) diamond burr combined with the protective sheath of the cutting burr. Configurations (b) and (c) were compared in the present study

The surgical procedure for PE-MHL has been reported in detail previously ²¹ and is summarised here. Briefly, PE-MHL was performed through a lateral approach under full endoscopic visualisation. The correct intervertebral level (L4–L5) was identified, marked with needles and confirmed fluoroscopically in dorsoventral and lateral projections. After confirmation, a guide wire was inserted through the laterally placed 18 G needle, which was then removed. A small skin incision was made, and a dilation sleeve was advanced over the guide wire before the guide wire was withdrawn. The operating sheath was inserted over the dilation sleeve, which was subsequently removed. After insertion of the endoscope, the intervertebral foramen was visualised and laminar bone removal was performed as previously described. ²¹

Each cadaver underwent two PE-MHL procedures at the level of L4–L5, yielding a total of 24 surgeries. The diamond burr was used on one side and the cutting burr on the opposite side, both combined with the protective sheath of the suction burr. The side of initial burr use was alternated between cadavers: the first cat underwent PE-MHL with the diamond burr on the left and the cutting burr on the right, whereas the order was reversed in the next specimen. This sequence continued throughout the study. To minimise the effect of instrument wear, new diamond and cutting burrs were used after every three procedures. To obtain comparable bone windows, approximately 30% of the lamina was planned for removal. The dissector width (2.8 mm) served as an intraoperative reference for the extent of laminar resection.

Assessment

Each surgical procedure was continued until the planned extent of bone removal was achieved and the spinal cord and corresponding nerve root were clearly visualised. Any intraoperative complications or technical adjustments were recorded.

The two burr types were compared based on the duration (mins) required to achieve spinal cord exposure, representing the time necessary for bone window creation.

Postoperatively, CT of the lumbar spine (L3–L6) was performed using a 64-slice CT scanner (SOMATOM Definition AS; Siemens) at 300 mAs, 120 kV and 0.6 mm slice thickness. The area of the bone window (mm²) was measured in the sagittal plane of the postoperative CT images. The bony margin corresponding to the surgical window was manually outlined, and the enclosed area was automatically calculated by the imaging software (Somaris/7 syngo CT VA48A) as described previously. ²¹ In addition, the laminar lengths of L4 and L5 (mm) were determined on pre- and postoperative images. These measurements were compared to calculate the laminar percentage removed for each burr type and vertebral level.

Finally, the effect of surgical side (left vs right) on surgical duration, bone window area and percentage of lamina removed were analysed to identify potential procedural or ergonomic effects.

Statistical analysis

All statistical analyses were performed using R software (version 4.5.0). Data were assessed for normality using the Shapiro–Wilk test and for homogeneity of variances using Levene’s test. As both assumptions were satisfied for all variables, group comparisons were conducted using Student’s t-tests. Complementary Bayesian analyses were performed to quantify the strength of evidence in favour of the alternative hypothesis, expressed as Bayes factors (BF₀₁) and posterior 95% credible intervals. All numeric results are presented as mean ± SEM. P ⩽0.05 was considered statistically significant.

Results

A total of 12 skeletally mature DSH cat cadavers (six female and six male) with a mean body weight of 3.46 ± 0.12 kg (range 2.5–4.4) were included in the study.

Surgical duration

The variable operated side (left vs right) did not significantly affect surgical duration, bone window area or the percentage of lamina removed (P >0.05 for all comparisons). Accordingly, data were pooled across sides for subsequent analyses.

The mean surgical duration was significantly shorter with the diamond burr (20.85 ± 1.44 mins) than with the cutting burr (25.62 ± 1.49 mins; P = 0.03). The estimated difference (4.77 mins, 95% confidence interval [CI] 0.4–9.1) corresponded to a large effect size (Hedges’ g = 0.91) (Figure 2).

Figure 2

Comparison of operation time (mins) required for spinal cord exposure during percutaneous endoscopic mini-hemilaminectomy using diamond and cutting burrs (n = 12 per group). Student’s t-test showed the diamond burr group to have significantly shorter operation times (P = 0.03) with large effect size (Hedges’ g = 0.91). A Bayesian two-sample t-test (Jeffreys-Zellner-Siow prior, r = 0.71) yielded log_e(Bayes factor [BF]₀₁) = −0.83 (BF₁₀ ≈ 2.3), providing anecdotal evidence for a difference; the posterior mean difference was 3.67 mins (95% equal-tailed interval –0.19 to 7.89, positive values favour cutting − diamond), which we consider a large effect. Bars represent mean ± SEM. P ⩽0.05 was considered statistically significant

To further substantiate these findings, a detailed statistical analysis was conducted to verify the robustness of the difference in operation time between burr types.

Statistical evaluation of operation time

The Student’s two-sample t-test was conducted to compare the mean operation times between the diamond and cutting burr groups (n = 12 per group; total n = 24). Assumptions of normality were met for both groups, as indicated by non-significant Shapiro–Wilk tests (diamond: P = 0.278; cutting: P = 0.182), while Levene’s test confirmed homogeneity of variances (P = 0.733), validating the use of the standard Student’s t-test. The standardised effect size for surgical duration was large (Hedges’ g = 0.91, 95% CI 0.08–1.72).

Complementary Bayesian analysis supported this finding, with a Bayes factor of BF₀₁ = exp(–0.83) ≈ 0.44 providing moderate evidence in favour of the alternative hypothesis. The posterior difference (diamond – cutting) was delta = –4.64 (95% CI –8.91 to −0.35), which excludes zero and corroborates the frequentist outcome of significantly reduced operation time when using the diamond burr.

Overall, these results consistently demonstrated a significantly shorter operation time for the diamond burr across both frequentist and Bayesian analyses, confirming the reliability of this finding (Figure 2).

CT evaluation

The mean bone window area did not differ significantly between the diamond burr (68.42 ± 2.02 mm²) and the cutting burr (63.92 ± 3.19 mm²; P = 0.27). The estimated difference (4.50 mm², 95% CI −3.0 to 11.9) corresponded to a small effect size (Hedges’ g = –0.47).

At the L4 level, the mean percentage of lamina removed was significantly larger with the diamond burr (39.18 ± 2.75%) than with the cutting burr (30.57 ± 2.14%; P = 0.02).

At the L5 level, the mean percentage of lamina removed was not significantly different with the diamond burr (32.03 ± 1.98%) or with the cutting burr (27.75 ± 2.30%; P = 0.17).

Except for the L4 diamond burr group, which slightly exceeded the planned 30% target (mean 39.18 ± 2.75%), all values were close to the intended range, resulting in an overall mean of 32.38 ± 2.31% across all burrs and vertebral levels (Table 1).

Table 1

Percentage of lamina removed at L4 and L5 after percutaneous endoscopic mini-hemilaminectomy performed with diamond and cutting burrs (n = 12 per group)

Burr	L4 (%)	L5 (%)	Overall mean
Diamond	39.18 ± 2.75	32.03 ± 1.98	35.61 ± 2.40
Cutting	30.57 ± 2.14	27.75 ± 2.30	29.16 ± 2.22
Overall mean	34.88 ± 2.46	29.89 ± 2.15	32.38 ± 2.31

Data are mean ± SEM. The overall mean represents the mean across burr types and vertebral levels

Intraoperative observations

All 24 procedures were completed successfully, and clear visualisation of the spinal canal, spinal cord and nerve root was achieved. No macroscopic evidence of nerve root or spinal cord trauma was observed in any of the 24 procedures. Replacement of the burrs after every third procedure did not subjectively indicate detectable wear. Both burr types produced comparable bone windows. The diamond burr generated smooth bone margins and allowed precise handling, facilitating controlled gliding along the laminar margin and accurate depth assessment owing to its rounded tip design. In contrast, the cutting burr produced more irregular, ridged bone edges that required frequent repositioning of the burr. The elongated, cylindrical design of the cutting burr made depth control more challenging. Overall, the cutting burr required increased pressure owing to the irregular bone surfaces it produced. In addition, greater instrument angulation was needed to keep the tip and middle portion of the burr in controlled contact with the bone during endoscopic visualisation. Three cutting burrs broke during the procedures, whereas no mechanical failures occurred with the diamond burr.

Discussion

This study evaluated the effect of burr type on surgical performance during PE-MHL in feline cadavers. Surgical duration was significantly shorter when using the diamond burr than when using the cutting burr, leading to the rejection of the initial hypothesis that the cutting burr would reduce procedure time during PE-MHL.

Interestingly, the mean surgical duration was approximately 5 mins shorter with the diamond burr (20.85 ± 1.44 mins) than with the cutting burr (25.62 ± 1.49 mins).

Although cutting burrs are generally regarded as more aggressive and capable of removing larger bone volumes within a shorter period,^22,23 the present results demonstrated higher efficiency with the diamond burr under endoscopic conditions. Intraoperatively, both burr types achieved adequate spinal canal exposure and clear visualisation of the spinal cord and corresponding nerve root. Moreover, the diamond burr produced smoother bone margins and provided more precise tactile feedback, facilitating navigation near delicate neural structures. In contrast, the cutting burr generated irregular bone surfaces and required greater caution near delicate neural structures, which may have prolonged the procedure. The differences in bone removal characteristics resulted in distinct handling properties. These differences likely increased mechanical load during instrument manipulation and may explain the higher incidence of instrument failure observed with the cutting burr. No mechanical failures occurred with the diamond burr, further supporting its superior handling characteristics and structural reliability under endoscopic conditions.

A round or ball-cutting burr tip may have improved handling characteristics compared with the cylindrical design evaluated in the present investigation. However, according to the results of the present study, a round cutting burr tip potentially would not have improved their cutting performance. As a ball-cutting burr tip is not available for the SpineTIP system, this difference could not be assessed in this study. Moreover, studies in dogs using full-endoscopic systems exclusively used ball-diamond burr tips or various burr types,^10,11,13 impeding our ability to draw further potential conclusions. Nevertheless, the findings of the present study support the recommendation of the manufacturer of the SpineTIP system established for humans for the application of diamond burrs for the use for mini-hemilaminectomy in cats. Given the anatomical constraints of the endoscopic surgical field and the proximity to the spinal cord and nerve roots, controlled handling and precision are critical. Overall, the stable cutting characteristics of the diamond burr may not only explain its shorter procedural time but also its better suitability for drilling in these sensitive regions. Therefore, regarding the cutting performance, the controlled handling and the decreased surgical duration observed in this study, the diamond burr may be recommended for PE-MHL in cats over the cutting drill available for the SpineTIP system.

At the L4 level, the diamond burr produced a significant greater bone window compared with the cutting burr. This finding cannot be explained by the authors. A previous experimental study evaluated the surgical exposure achieved using two sizes of an integrated endoscopic system for thoracolumbar hemilaminectomy in dogs. ¹⁵ In this study, the caudal extent of the hemilaminectomy was significantly longer compared with the cranial extent, leading to the conclusion that herniations extending approximately one-third cranially and two-thirds caudally from the intervertebral disc space could be approached with the system used. ¹⁵ The authors hypothesised that the anatomical position of the initial cannula placement affected by the articular processes may have influenced the cranial and caudal extent of the hemilaminectomy. ¹⁵ In addition, the surgical site appeared to affect the extent of the caudal window dimensions, although this may have represented a type I error. ¹⁵ Overall, the findings of Drury et al ¹⁵ and those of the present study suggest that both breed-specific anatomical differences and characteristics of the respective endoscopic systems may influence surgical exposure. In light of our results, further investigations evaluating the achievable dimensions of PE-MHL at different vertebral levels are warranted to establish recommendations for the SpineTIP system in cats, particularly regarding surgical planning in relation to the extent of disc material migration.

Several limitations should be acknowledged. As a cadaveric study, physiological factors such as bleeding, tissue elasticity and healing response could not be replicated. Consequently, intraoperative safety and postoperative outcomes under clinical conditions cannot be inferred. Neither histopathological evaluation nor postoperative MRI was performed to assess potential injury to the nerve roots or the spinal cord, which limits the evaluation of possible iatrogenic trauma. Moreover, only one spinal segment (L4–L5) was examined, and anatomical variation at other lumbar levels may influence the applicability of the results.

Finally, each burr was used for three consecutive procedures owing to budget limitations. Potential effects of gradual wear were not analysed statistically and may limit the interpretation of the results, although no subjective performance differences were observed. These conditions were consistent across groups. An experimental study evaluating the cutting rates of various diamond drills on ceramic material reported approximately 20% wear over 20 procedures. ²⁵ Although these results are not directly comparable, they suggest minimal wear within three procedures, as performed in the present investigation. Future studies should assess the influence of diamond burr wear on surgical duration.

Conclusions

Both burr types were effective for PE-MHL in cats. However, the diamond burr allowed more efficient, precise and controlled bone removal, resulting in smoother bone margins and consequently a significantly shorter surgical duration. These findings indicate that the diamond burr is a more reliable instrument for controlled and safe bone removal during PE-MHL and may be recommended over the cutting burr for this purpose. Further studies are warranted to confirm these results under clinical conditions and at different spinal levels.