The Current Testing Protocols for Biomechanical Evaluation of Lumbar Spinal Implants in Laboratory Setting: A Review of the Literature

Table 2

Summary of explanatory variables examined for all in vitro biomechanical investigations.

	Single FSU Studies		Multisegmental Studies
	Number of articles	References	Number of articles	References
Number of FSUs
Single (1)	9	[22–30]	—
Two or more	—		46	[31–76]
Two (2)	—		5	[38, 42, 43, 60, 66]
Three (3)	—		13	[33, 34, 36, 47–49, 52–55, 67, 68, 74]
Four (4)	—		12	[31, 40, 46, 50, 52, 56, 59, 61, 69–71, 76]
Five (5)	—		13	[32, 35, 39, 41, 44, 45, 51, 57, 63–65, 73, 75]
Six (6)	—		13	[37, 62, 72]
Operative Level^*
T12-L1	2	[26, 30]	—
L1-L2	3	[26, 29, 30]	1	[59]
L2-L3	7	[22, 23, 25–27, 29, 30]	6	[50, 57, 58, 61, 73, 76]
L3-L4	4	[24, 26, 29, 30]	27	[31, 32, 34, 40, 41, 45, 47–51, 54, 57, 58, 61–65, 67–69, 72–76]
L4-L5	7	[22, 25–30]	27	[31, 33–37, 39, 41, 43, 44, 46, 52, 53, 55–58, 60–64, 66, 67, 70, 71, 73]
L5-Sacrum	1	[30]	9	[31, 37–39, 42, 43, 60, 71, 73]
Number of specimens
Less than six (6)	1	[24]	4	[34, 46, 54, 69]
Six (6) or seven (7)	2	[22, 23]	29	[31–33, 36–39, 42–45, 52, 53, 55, 57, 59–63, 66–68, 70–75]
More than seven (7)	6	[25–30]	13	[35, 40, 41, 47–51, 56, 58, 64, 65, 76]
Loading protocol^*
Flexibility	9	[22–30]	37	[31, 33, 34, 36–38, 40, 42–54, 57–61, 63–66, 68–70, 72–76]
Hybrid	—		8	[35, 39, 41, 55, 56, 62, 67, 70]
Other	—		2	[32, 60]
Preload^*
None (0 N)	2	[23, 27]	14	[31, 34, 40, 47–49, 51, 52, 62, 66–68, 71, 74]
Axial	1	[22]	2	[42, 66]
Follower	—		8	[45, 46, 50, 56, 69–71, 76]
Less than 400 N	—		1	[71]
400 N	—		7	[45, 46, 50, 56, 69, 70, 76]
More than 400 N	—		—
Follower FE only	1	[27]	10	[44, 47–49, 53, 59–62, 65]
Less than 400 N	—		—
400 N	—		6	[44, 47–49, 61, 65]
More than 400 N	1	[27]	1	[62]
Bending-compression	1	[29]	2	[34, 52]
Unspecified^**	5	[24–26, 28, 30]	21	[32, 33, 35–39, 41, 43, 53–55, 57–60, 63, 64, 72, 73, 75]
Torque applied^*
Less than 7.5 Nm	4	[24–26, 29]	10	[31, 40, 44, 54, 58, 61–63, 65, 72]
7.5 Nm	4	[22, 23, 28, 30]	22	[33, 37–39, 42, 43, 46–53, 59, 60, 64, 66, 67, 74–76]
More than 7.5 Nm	1	[27]	17	[34–36, 41, 44, 45, 55–58, 61, 65, 68–71, 73]
Displacement control	—		1	[32]
Loading type and load rate
Dynamic loading^*	9	[22–30]	26	[31, 32, 37, 39, 41–46, 48, 51, 55–57, 61–63, 65, 66, 68–71, 74, 75]
Less than 1 deg/sec	2	[23, 24]	2	[32, 45]
1–3 deg/sec	3	[23, 25, 28]	12	[31, 32, 39, 41, 55, 57, 61, 63, 66, 68, 71, 74]
4 deg/sec	—		1	[51]
Less than 0.1 Nm/sec	—		2	[37, 75]
0.1–0.5 Nm/sec	3	[22, 26, 27, 30]	6	[42, 43, 46, 56, 69, 70]
Other	1	[29]	—
Unspecified	—		4	[44, 48, 62, 65]
Quasistatic loading	—		10	[33, 34, 36, 38, 52, 58–60, 64, 73]
1.0 Nm/sec	—		1	[36]
1.5 Nm/sec	—		6	[33, 38, 52, 59, 60, 64]
2.0 Nm/sec	—		2	[58, 73]
Unspecified	—		1	[34]
Unspecified^***	—		10	[35, 40, 48–50, 53, 54, 67, 72, 76]
Precondition Cycles
One (1)	—		3	[41, 47, 48]
Two (2)	7	[22–26, 29, 30]	23	[31, 32, 37, 39, 40, 42, 43, 46, 55–57, 62, 63, 66–75]
Three (3) or more	2	[27, 28]	9	[33, 34, 36, 38, 52, 58–60, 64]
Unspecified	—		8	[35, 45, 49, 50, 53, 54, 61, 76]
Until 2 Reproducible cycles	—		3	[44, 51, 65]
Simulated Injury/Instrumentation
No Instrumentation	1	[25]	2	[49, 61]
Instrumentation	8	[22–24, 26–30]	44	[31–48, 50–60, 62–76]
Injury Described	8	[22–24, 26–30]	28	[31, 33–41, 43–46, 48, 54–58, 60, 65–67, 69, 70, 74, 75]
Injury Tested	3	[23, 24, 27]	15	[33–37, 39, 40, 44, 45, 54, 55, 57, 66, 67, 69]
Common Variables Measured
Range of Motion	9	[22–30]	46	[31–76]
Neutral Zone	5	[22, 23, 25, 26, 29]	10	[33, 38, 45, 58–60, 65–67, 73]
Elastic Zone	—		4	[33, 38, 59, 60]
Intradiscal Pressure	—		9	[36, 39, 41, 46, 50, 55, 66, 69, 70]
Stiffness	3	[22, 25, 28]	5	[50, 58, 65, 68, 76]
Neutral Zone Stiffness	1	[22]	1	[65]
Elastic Zone Stiffness	1	[22]	1	[58]
“Other” Stiffness	2	[25, 28]	3	[50, 68, 76]
Radiological Measurements^****	2	[22, 30]	5	[44, 52, 53, 59, 74]
Bone Strain	—		4	[32, 46, 60, 70]
Instantaneous Axis of Rotation (IAR)	—		4	[33, 36, 59, 60]
Bone Mineral Density	5	[22, 26, 28–30]	14	[31, 38, 42, 43, 45, 46, 53, 56, 58–60, 64, 67, 73]

Some articles appear more than once under this classification due to the use of more than one testing protocol.
^**The application of an axial preload was not mentioned hence articles in this category were distinguished from those that stated explicitly the use of 0 N of axial preload.
^***Includes articles that mentioned to have followed a previously described protocol without providing the information in the methodology of the actual article.
^****Articles describing manual measurements (i.e caliper measurements) were excluded from this category.