Potential contribution of pedicle screw design to loosening rate in patients with degenerative diseases of the lumbar spine: An observational study

Table 1 Characteristics of the study patients, n (%)

Characteristic	Patients, n (%)
Number of levels fused
1 level	115 (65.7)
2 levels	47 (26.9)
3 levels	16 (9.1)
4 levels	1 (0.6)
5 levels	1 (0.6)
Fusion type
Posterior fusion only	25 (14.3)
TLIF	150 (85.6)
Amount of decompression
No resection of posterior structures	18 (10.3)
Laminectomy	46 (26.3)
Unilateral total facet joint removal	108 (61.7)
Bilateral total facet joint removal on at least one level	46 (26.3)
Bone characteristics
Radiodensity, HU	M = 126.25 ± 3.20 SD = 42.31, range 43.17-282.07

HU: Hounsfield units; TLIF: Transforaminal lumbar interbody fusion.

Table 2 Characteristics of the screws used in this study

Design	External diameter (max)	Internal diameter (max)	Helical pitch	Type of thread	n (%)
cyl/cyl	6.5	4.0	3.0	Buttress	6 (3.4)
cyl/cyl	7.5	4.5	3.0	Buttress	9 (5.1)
cyl/con	6.5	4.0	2.5	Buttress	40 (22.9)
cyl/con	7.5	5.0	2.5	Buttress	4 (2.3)
con/con	6.0	4.0	3.0	V-shape	48 (27.4)
con/con	7.0	4.5	3.0	V-shape	5 (2.9)
con/con	6.0	4.5	2.5	V-shape	49 (28.0)
con/con	7.0	5.5	2.5	V-shape	14 (8.0)

con/con: Conical core and thread; cyl/con: Cylindrical thread and conical core; cyl/cyl: Cylindrical core and thread.

Table 3 Parameters included in the general logistic regression model

Parameter	Regression coefficient	OR per unit change with 95%CI
Intercept	−4.819065; Р = 0.3617	-
Radiodensity, HU	−0.0271586; P < 0.0001	0.9732 (0.9623-0.9842)
Number of fused levels	1.111651; P < 0.0001	3.0394 (1.7566-5.2590)
Bilateral total facet joints removal (at least one level)	1.124871; Р = 0.4359	3.0798 (1.0023-9.2781)
Laminectomy	0.4418158; Р = 0.5333	1.5555 (0.5090-4.7533)
Posterior fusion performed without interbody fusion	−0.4247137; Р = 0.4550	0.6540 (0.2128-2.0097)
Maximal external screw diameter	−5.966555; Р = 0.3798	0.0026 (0.000009-0.7153)
Maximal internal screw diameter	6.430714; P = 0.0253	620.61 (2.2363-172234.6)
Helical pitch	4.611687; Р = 0.0081	100.6538 (3.3668-3009.1310)
Thread type (buttress vs V-shape)	−1.034237; Р = 0.9949	0.1070 (0.0090-1.2659)
Design cyl/cyl	0.1446205; P = 0.8568	1.1566 (0.2378-5.6156)
Design cyl/con	−1.731799; Р = 0.2767	0.1769 (0.0077-4.0602)
Design con/con	0.2718265; P = 0.7594	1.3124 (0.2282-7.5451)

CI: Confidence interval; con/con: Conical core and thread.; cyl/con: Cylindrical thread and conical core; cyl/cyl: Cylindrical core and thread; HU: Hounsfield unit; OR: Odds ratio.