Development, reliability, and piloting of a wheelchair caster failure inspection tool (C-FIT)

Figures & data

Figure 1. Caster failure modes commonly seen in the field.

Figure 2. Flowchart of development and testing activities for C-FIT.

Figure 3. Failed caster samples used in test-retest reliability testing study.

Table 1. Caster failure modes reported in wheelchair testing studies and field trials.

Caster Part	Failure Mode
Axle bearing	Corrosion* Obstruction to rolling* Fracture* Play between bearings and axle bolt Loss of contact with wheel Loss of trueness of the bearing Dirt contamination
Axle bolt	1. Fracture
Caster Wheel	Fracture* Corrosion*
Tire	Tire worn-out* Tread worn-out* Cracking* Deflated* Tire etching on sides Pitting
Stem Bearing	Corrosion* Play between bearings and stem bolt Dirt contamination
Stem Bolt	1. Fracture*
Fork	Bent* Fracture* Corrosion*
Fasteners	Corrosion Loose fasteners

* Failure selected for C-FIT inclusion.

Table 2. Face validity testing results.

Caster failure mode	Score for inclusion (%)	Risk for user consequences (%)		Risk for failure of other wheelchair parts (%)
		Medium Risk – High Risk	No Risk – Low Risk	Medium Risk – High Risk	No Risk – Low Risk
Axle bearing corrosion	100	20	80	20	80
Axle bearing's obstruction to rolling	100	20	80	20	80
Axle bearing fracture	100	60	40	80	20
Caster wheel fracture	100	60	40	80	20
Tire worn out	100	0	100	0	100
Deflated tire	100	0	100	40	60
Stem bolt fracture	100	100	0	100	0
Bent fork	100	60	40	60	40
Tire cracking	80	20	80	40	60
Stem bearing corrosion	80	40	60	40	60
Fork fracture	80	100	0	100	0
Fork corrosion	80	25	75	25	75
Caster wheel corrosion	60	20	80	40	60
Tire tread worn out	40	0	100	0	100

Table 3. Demographic characteristics of test-retest reliability testing study participants.

Demographic	Physical inspection group	Online inspection group
Experience with wheelchairs (in years)	3.3 ± 2.4	9.4 ± 5.3
Professions (multiple choice response)
• Engineer	11	5
• Physician	1	2
• Clinician	0	5
• Therapist	0	3
• Designer	3	4
• Manufacturer	0	3
• Technician	1	3
• Other	2 (Researchers)	3 (ATP, Manager, Marketing)
Experience with servicing wheelchairs (Yes/No)	75% Yes, 25% No	100% Yes
Raters that completed test session	12	13
Number of raters who completed the study	12	11
Retest interval	2 weeks (n = 11), 2 weeks and 4 days (n = 1)	2.4 ± 0.6 weeks

Table 4. C-FIT used for reliability testing.

Failure mode	Failure present	Failure not present	Unable to evaluate
Axle bearing corrosion
Axle bearing obstruction to rolling
Axle bearing fracture
Caster wheel fracture
Tire roll-off
Tire worn-out
Tire cracking
Deflated tire
Stem bearing fracture
Stem bearing corrosion
Stem bolt fracture
Bent fork
Fork fracture
Fork corrosion

Table 5. Test-retest reliabilities for the physical inspection group.

Failure modes	Range values		Kappa agreement (% participants)
	% agreement	Kappa	Poor	Fair	Moderate	Substantial	Almost perfect
Axle bearing corrosion	71.43–100.00	0.56–1.00	0	0	25	0	75
Axle bearing's obstruction to rolling	50.00–100.00	0.14–1.00	16.67	8.33	8.33	50	16.67
Axle bearing fracture	32.14–96.43	0.11–0.90	25	16.67	8.33	33.33	16.67
Caster wheel fracture	78.57–100.00	0.45–1.00	0	0	8.33	25	66.67
Tire roll-off	96.43–100.00	0.78–1.00	0	0	0	50	50
Tire worn out	75.00–96.43	0.46–0.93	0	0	25	50	25
Tire cracking	75.00–100.00	0.38–1.00	0	8.33	25	16.67	50
Deflated tire	67.86–100.00	0.24–1.00	0	8.33	16.67	33.33	41.67
Stem bearing fracture	50.00–92.86	0.25–0.88	0	16.67	16.67	50	16.67
Stem bearing corrosion	67.86–100.00	0.48–1.00	0	0	16.67	25	58.33
Stem bolt fracture	71.43–96.43	0.52–0.94	0	0	8.33	25	66.67
Bent fork	85.71–100.00	0.72–1.00	0	0	0	25	66.67
Fork fracture	82.14–100.00	0.52–1.00	0	0	8.33	8.33	83.33
Fork corrosion	78.57–100.00	0.67–1.00	0	0	0	16.67	83.33

Table 6. Test-retest reliabilities for the online inspection group.

Failure modes	Range values		Kappa agreement (% participants)
	% agreement	Kappa	Poor	Fair	Moderate	Substantial	Almost perfect
Axle bearing corrosion	53.57–96.43	0.37–0.93	0	9.09	9.09	54.55	27.27
Axle bearing's obstruction to rolling	39.29–92.86	0.14–0.87	9.09	45.45	18.18	18.18	9.09
Axle bearing fracture	25.00–92.86	0.12–0.68	54.55	18.18	18.18	9.09	0
Caster wheel fracture	50.00–100.00	0.00–1.00	9.09	27.27	36.36	9.09	18.18
Tire roll-off	78.57–100.00	0.39–1.00	0	9.09	18.18	36.36	36.36
Tire worn out	67.86–92.86	0.14–0.76	9.09	18.18	18.18	54.55	0
Tire cracking	67.86–96.43	0.35–0.86	0	9.09	54.55	18.18	18.18
Deflated tire	78.57–100.00	0.45–1.00	0	0	9.09	45.45	45.45
Stem bearing fracture	53.57–96.43	0.01–0.76	27.27	18.18	18.18	36.36	0
Stem bearing corrosion	64.29–92.86	0.30–1.00	0	27.27	9.09	45.45	18.18
Stem bolt fracture	42.86–100.00	0.15–1.00	9.09	9.09	18.18	45.45	18.18
Bent fork	35.71–100.00	0.04–0.93	18.18	27.27	18.18	0	18.18
Fork fracture	42.86–100.00	0.18–1.00	9.09	0	36.36	18.18	36.36
Fork corrosion	42.86–96.43	0.29–0.94	0	18.18	18.18	9.09	54.55

Table 7. Interrater reliabilities for the test and retest sessions of the physical inspection group.

	Test session			Retest session
Failure modes	Kappa	%agreement	Accuracy (%)	Kappa	%agreement	Accuracy (%)
Axle bearing corrosion	0.712 ± 0.019	89.29	77.38 ± 5.12	0.708 ± 0.019	89.88	78.87 ± 4.24
Axle bearing's obstruction to rolling	0.367 ± 0.018	82.14	62.80 ± 11.24	0.406 ± 0.019	80.36	60.12 ± 9.65
Axle bearing fracture	0.182 ± 0.018	74.70	66.67 ± 15.73	0.386 ± 0.018	84.23	72.32 ± 10.83
Caster wheel fracture	0.831 ± 0.022	98.21	98.21 ± 2.73	0.775 ± 0.021	97.02	97.02 ± 5.02
Tire roll-off	0.802 ± 0.022	97.62	97.02 ± 1.97	0.887 ± 0.023	98.81	98.81 ± 1.68
Tire worn out	0.438 ± 0.024	83.33	77.38 ± 10.94	0.381 ± 0.023	81.25	77.98 ± 14.49
Tire cracking	0.617 ± 0.023	89.88	89.29 ± 6.36	0.56 ± 0.024	89.58	89.58 ± 6.59
Deflated tire	0.305 ± 0.019	91.67	91.67 ± 12.90	0.511 ± 0.022	94.35	94.35 ± 10.9
Stem bearing fracture	0.485 ± 0.017	77.08	73.21 ± 13.95	0.542 ± 0.017	80.06	74.70 ± 12.67
Stem bearing corrosion	0.693 ± 0.017	86.61	85.42 ± 13.24	0.715 ± 0.017	88.99	86.31 ± 12.86
Stem bolt fracture	0.665 ± 0.018	89.29	87.50 ± 9.73	0.726 ± 0.018	91.07	90.18 ± 9.57
Bent fork	0.77 ± 0.022	93.15	93.15 ± 10.25	0.75 ± 0.021	93.45	93.45 ± 10.60
Fork fracture	0.771 ± 0.019	92.86	92.86 ± 10.31	0.817 ± 0.019	93.15	93.15 ± 9.16
Fork corrosion	0.758 ± 0.017	90.77	90.77 ± 9.83	0.811 ± 0.018	91.96	91.96 ± 9.00

Table 8. Interrater reliabilities for the test and retest sessions of the online inspection group.

	Test session			Retest session
Failure modes	Kappa	%agreement	Accuracy (%)	Kappa	%agreement	Accuracy (%)
Axle bearing corrosion	0.393 ± 0.019	69.48	60.71 ± 21.80	0.412 ± 0.019	80.71	51.30 ± 19.93
Axle bearing's obstruction to rolling	0.188 ± 0.021	60.71	43.18 ± 26.79	0.152 ± 0.021	69.29	29.55 ± 24.37
Axle bearing fracture	0.081 ± 0.018	58.77	50.97 ± 23.20	0.042 ± 0.023	66.23	40.91 ± 23.76
Caster wheel fracture	0.356 ± 0.021	84.09	79.55 ± 4.84	0.564 ± 0.021	90.58	85.39 ± 7.98
Tire roll-off	0.479 ± 0.022	94.81	94.81 ± 7.20	0.742 ± 0.024	96.10	96.10 ± 5.15
Tire worn out	0.209 ± 0.023	76.62	74.68 ± 22.40	0.381 ± 0.026	81.82	79.22 ± 15.96
Tire cracking	0.448 ± 0.022	84.09	84.09 ± 9.06	0.454 ± 0.024	88.31	88.31 ± 8.76
Deflated tire	0.677 ± 0.022	96.10	96.10 ± 3.87	0.953 ± 0.027	98.05	98.05 ± 3.18
Stem bearing fracture	0.131 ± 0.023	65.91	62.34 ± 16.43	0.193 ± 0.022	64.94	60.71 ± 21.10
Stem bearing corrosion	0.251 ± 0.2	67.21	62.34 ± 18.78	0.335 ± 0.022	67.86	62.34 ± 24.57
Stem bolt fracture	0.189 ± 0.024	69.81	69.16 ± 14.94	0.403 ± 0.022	73.38	71.43 ± 17.50
Bent fork	0.186 ± 0.026	67.86	67.86 ± 18.71	0.416 ± 0.025	82.79	79.87 ± 15.48
Fork fracture	0.341 ± 0.022	72.73	72.73 ± 14.94	0.575 ± 0.021	84.42	84.42 ± 14.06
Fork corrosion	0.409 ± 0.02	76.95	75.00 ± 13.54	0.594 ± 0.19	80.84	77.27 ± 16.84

Table 9. Feedback by study participants regarding usability.

Feedback	Physical inspection group (n = 12)	Online inspection group (n = 11)
The caster anatomy information was redundant in this survey	0	5
The instructions for evaluating casters were helpful	8	10
I need more training materials before using C-FIT	1	1
C-FIT is easy to use for rating caster failures	8	7
Evaluating the casters through photos was difficult	NA	5
I would like to use C-FIT for collecting failure data on casters	NA	4
I prefer using C-FIT online through a laptop, phone or tablet for collecting failures	NA	7
I prefer using a paper version of C-FIT for collecting failures	NA	3

Table 10. Individual feedback and related revisions to C-FIT.

No.	Inspection issues	Revisions
1.	Participants were not sure which option to choose when parts were missing.	Missing parts was added as an option.
2.	In many cases, bearings were not visible and so, the participants guessed the failure based on condition of the caster. Broken bearing seals were not scored as failures.	Instructions were updated to evaluate bearings and bearing seals in detail and it was noted that participant should check the ‘Unable to evaluate’ option if the detailed inspection is not carried out.
3.	One participant considered loose washers are part of the bearings and scored it as a fracture failure. In some cases, failed bearings were left in the bags and not evaluated at all.	The instructional materials note that the participant should go through the informational materials prior to caster inspections and check each part.
4.	Failures of tire worn-out and cracking were confusing to participants. Some participants rated tread wear as tire worn-out.	Instructions for tire failures were described in detail to explain differences in tire worn-out, tread worn-out and cracking.
5.	Any dirt on the tire was rated as tire wear	Instructions that dirt on tire do not account for wear were added.
6.	When inspecting for tire deflated failure, several participants did not check if the tire was pneumatic.	Instructions were added for checking the pneumatic tire by looking for a valve prior to rating the failure.
7.	Corrosion and obstruction to rolling failures were found to be subjective. Participants were ambiguous about what degree of corrosion and obstruction should be rated as failure.	Corrosion was divided into mild and high corrosion as the failure modes. Both failure modes were distinguished based on the outcomes of the failure inspection. The obstruction to rolling failure was changed to sticky bearing failure for axle bearings.

Figure 4. Field failures collected using C-FIT.

Figure 5. Failures with Harmony wheelchair casters.

Figure 6. Failures with Whirlwind Roughrider (left), Sunrise Quickie Rumba (center) and Invacare Mirage (right) casters.