Would loss to follow-up bias the outcome evaluation of patients operated for degenerative disorders of the lumbar spine?

Figures & data

Figure. Study population.

Table 1. Characteristics of the study population

	Alln= 633	IRespondentsn =491	IINon-respondentsn= 142	III ^aConsistent non-respondentsn = 12	p-value of difference
					I vs. II	I vs. III
Age, median (95% CI)	42 (41–44)	43 (41–44)	40 (37–44)	34 (29–51)	0.04	0.04
Females (%)	233 (36.8)	187 (38.1)	46 (32.4)	4(33.3)	0.2	0.7
BMI, median (95% CI) kg/m^2 b	26 (25–26)	26 (25–26)	26 (25–26)	25 (22–27)	0.7	0.5
Smokers (%)	296 (47)	224 (46)	71 (50)	8 (67)	0.4	0.2
Living alone, n (%)	171 (27)	126 (26)	45 (32)	8 (67)	0.2	0.002
Had child less than 8 years, n (%)	178 (28)	133 (27)	45 (32)	3 (25)	0.3	0.9
Weeks on sick leave, median (95% CI)	8 (5–11)	8 (6–12)	4 (1–13)	1 (0–24)	0.1	0.2
Days of hospital stay, median (95%CI)	4 (4–4)	4 (4–4)	3 (3–4)	3 (2–4)	0.02	0.1
Previous low back operation, n (%)	76.0 (12)	64 (13)	12.0 (9)	1 (8)	0.1	0.6
Had complication to the surgery, n (%)	31 (5)	29 (6)	2 (1)	0	0.03	0.4
EQ-5D score, median (95% CI)	0.16 (0.12–0.19)	0.16 (0.09–0.19)	0.16 (0.09–0.33)	0.23 (-0.10–0.69)	0.4	0.9
Health state, median (95% CI)	40 (35–40)	40 (35–40)	43 (38–50)	36 (22–55)	0.08	0.9
Leg pain, median (95% CI)	67 (64–69)	68 (65–70)	65 (60–69)	59 (26–84)	0.5	0.6
Back pain, median (95% CI)	54 (50–60)	55 (50–61)	52 (45–57)	51 (45–83)	0.1	0.6
Were anxious and/or depressed, n (%) ^c	304 (48)	231 (47)	73 (51)	8 (67)	0.4	0.2
Educational level, n (%)					1.0 ^d	0.8 ^d
Primary school	189 (30)	148 (30)	41 (29)	5 (42)
Vocational school	218 (34)	168 (34)	50 (35)	3 (25)
Gymnasium / high school	74 (12)	60 (12)	14 (10)	1 (8)
University or college < 4 years	81 (13)	66 (13)	15 (11)	1 (8)
University or college > 4 years	71 (11)	49 (10)	22 (16)	2 (17)
Employment status, n (%)					0.7 ^e	0.8 ^e
On sick leave	345 (55)	276 (56)	69 (49)	6 (50)
On partial sick leave	33 (5)	24 (5)	9 (6)
Working full time	79 (13)	54 (11)	25 (18)	2 (17)
Homemaker	10 (2)	9 (2)	1 (1)
Student	28 (4)	12 (4)	7 (5)	1 (8)
Unemployed	9 (1)	7 (1)	2 (1)	1 (8)
Retired pensioner	62 (10)	53 (11)	9 (6)
On rehabilitation ^f	27 (4)	19 (4)	8 (6)
Disability pensioner	40 (6)	28 (6)	12 (9)	2 (17)
ASA grade I–V, n (%) ^g					0.8 ^h	0.4 ^h
Grade I	135 (40)	116 (36)	48 (40)	2 (22)
Grade II	189 (57)	192 (59)	69 (57)	7 (78)
Grade III	10 (3)	17 (5)	4 (3)

^a Group III was a subgroup of group II. ^b Body mass index. ^c Mild to severe problems. ^d University or college education? (yes/no). ^e On full or partial sick leave, on rehabilitation, or disability pensioner? (yes/no). ^f Patients having received worker's compensation for more than 12 months with the prospect of returning back to work, or permanent disability status. ^g No patients had ASA grade > III. ^h ASA grade I vs. grade II and III.

Table 2. Indications for and types of surgery among respondents non-respondents

Group	Alln = 633	IRespondentsn = 491	IINon-respondentsn = 130	III ^aConsistent non-respondentsn = 12
Indications for surgery, n (%)
Lumbar disc herniation	519 (82)	399 (81)	120 (84)	12 (100)
Central spinal stenosis	94 (15)	78 (16)	16 (11)
Lateral spinal stenosis	39 (6)	31 (6)	8 (6)
Segmental instability	31 (5)	24 (5)	7 (5)
Sum ^b	683	532	151	12
Types of surgery, n (%)
Microdiscectomy	476 (75)	362 (74)	114 (80)	11 (92)
Laminectomy	111 (17)	90 (18)	21 (15)
Instrumented fusion	30 (5)	24 (5)	6 (4)
Chemonucleolysis	16 (3)	15 (3)	1 (1)	1 (8)
Sum	633	491	142	12

^a Group III was a subgroup of group II. ^b Patients could have more than one indication for surgery.

Table 3. Sequential outcomes of the study population during 2 years of follow-up

Follow-up	3 months		12 months		2 years
n (response rate)	598 (95%)		574 (91%)		629 (99%) ^a
EQ-5D score ^b	0.45 (0.41–0.48)	< 0.001	0.46 (0.43–0.50)	< 0.001	0.46 (0.43–0.49)	< 0.001
Health state ^b	29 (27–31)	< 0.001	31 (29–34)	< 0.001	30 (28–32)	< 0.001
Leg pain ^b	43 (41–46)	< 0.001	41 (38–44)	< 0.001	41 (38–44)	< 0.001
Back pain ^b	29 (27–31)	< 0.001	28 (25–31)	< 0.001	27 (24–30)	< 0.001
Benefited from the operation, n (%) ^c	539 (90)		527 (92)		571 (91)
Received worker's compensation, n (%) ^d	335 (57)		166 (31)		181 (29)

^a Includes non-respondents interviewed by telephone.

^b Absolute values (improvements from baseline) are shown as mean change, (95% CI) and p-value

^c Patients who stated that they had “some”, “much”, or “very much” benefit from the operation.

^d Patients who were on full or partial sick leave, on rehabilitation, or disability pensioners.

Table 4. Subgroup analyses of respondents and non-respondents at 2 year

Outcome ^a	I. Respondentsn = 491(77.6%)		II. Non-respondentsn = 138(21.8%)		III. Consistentnon-respondents ^bn = 11		p-value of difference
							I vs. II	I vs. III
EQ-5D score ^c	0.46 (0.36–0.60)	< 0.001	0.41 (0.30–0.64)	< 0.001	0.64 (0.19–0.76)	0.003	0.8	0.6
Health state ^d	31 (28–34)	< 0.001	27 (22–32)	< 0.001	28 (13–43)	0.002	0.1	0.7
Leg pain ^d	40 (37–43)	< 0.001	44 (38–50)	< 0.001	43 (25–61)	0.001	0.3	0.8
Back pain ^c	22 (18–28)	< 0.001	26 (17–32)	< 0.001	40 (17–66)	0.008	1.0	0.3
Benefited from the operation, n (%) ^e	447 (91)		124 (91)		11 (100)		0.8	0.3
Received workers compensation, n (%) ^f	141 (29)4		0 (29)		4 (36)		1.0	0.6

^a Improvements from baseline (absolute values) are shown.

^b Group III is a subgroup of group II.

^c Median change, (95% CI) and p-value

^d Mean change, (95% CI) and p-value

^e Patients who stated that they had “some”, “much”, or “very much” benefit from the operation.

^f Patients who were on full or partial sick leave, on rehabilitation, or disability pensioners.

Table 5. Types of complications in 31 (5%) of 633 patients ^a

Complications	Alln = 633	I. Respondentsn = 491	II. Non-respondentsn = 142
Dural tear	9	9
Deep wound infection	5	5
Superficial wound infection	10	9	1
Urinary bladder infection	2	2
Reoperation within the same hospital stay	2	2
Intraoperative nerve root injury	1	1
Postoperative muscle hernia	1		1
Gall bladder infection	1	1
Deep leg vein thrombosis	1	1
Gastric ulcer hemorrhage	1	1
Minor myocardial infarction	1	1
Sum, n (%)	34 (6)	32 (7)	2 (1)

^a 3 of the patients had 2 complications. No complications occurred in consistent non-respondents (group III).

Table 6. Risk factors for failure to respond at 2 years of follow-up in 633 patients

Factors	OR ^a, b	95% CI	p-value	OR ^c	95% CI	p-value
Only received postal questionnaires at 2 years ^d	3.2	(2.1–4.9)	< 0.001	3.2	(2.1–4.9)	< 0.001
Age	0.99	(0.97–1.0)	0.05	0.98	(0.97–1.0)	0.05
Had any complication	0.23	(0.54–1.0)	0.05	0.26	(0.06–1.1)	0.07
Days of hospital stay	0.89	(0.82–0.96)	0.01	0.99	(0.90–1.1)	0.9
Health state	1.0	(0.99–1.0)	0.1
Living alone	1.3	(0.89–2.0)	0.2

^a OR: odds ratio. ^b Univariate analysis. ^c Multivariate analysis. ^d Protocol A, operated in 2000 and 2001.