Lower Extremity Injuries in Lateral Impact: A Retrospective Study

Abstract

A retrospective analysis of the NASS/CDS database from 1993 to 2000 was used to investigate lower extremity injury in lateral impact. The analysis includes the study of the injury patterns, crash characteristics and the interactions between the occupant and the vehicle interior, including injuries to the farside occupants. The findings include significantly different injury patterns for the nearside and farside impacts. In particular, while the proportion of pelvis/hip injuries, with respect to AIS2 and AIS3 lower extremity skeletal injuries and 2–4 and 10–8 o’clock side impacts, was higher in nearside (70.4%) than farside (38.3%), the opposite trend was observed for the thigh (2.8% vs 4.5%), knee (6.2% vs 16.7%), leg (10.1% vs 19.5%) and foot/ankle (5.6% vs 14.7) injuries. Analysis of the PDOF suggested that a large proportion the impacts occurred obliquely, at approximately 10 and 2 o’clock, with a rearward component of force. It is hoped that the findings of the current study can help to investigate injury mechanisms.

The lower extremity has been reported to be a commonly injured body region in side impact collisions [Thomas and Bradford, 1989, Fildes, Vulcan, Lane, et al., 1994]. Injuries are typically minor to serious (AIS1 to AIS3) but may result in long-term disability and impairment [Shoaf-Ore, Tanner, States 1993, MacKenzie, Siegel, Shapiro et al. 1988, Miller, Martin, Crandall 1995]. The anatomic structures, from the pelvis to the foot, were not examined in detail in previous lateral impact studies. This is important because most lateral impact biomechanical research concentrates only on the pelvic region and not the other structures of the lower extremity.

Three other studies have investigated field data from the UK and Germany to identify lower extremity injury patterns and the associated crash environment [Pattimore, Ward, Thomas et al., 1991, Otte, 1996, Thomas and Frampton, 1999]. The lower extremity injuries represented approximately one-third of all injuries in all impact modes studied (frontal, lateral and rear impacts). The incidence of AIS1 to AIS3 lower extremity injuries was greater in frontal impacts than lateral impacts [Pattimore, et al., 1991, Otte, 1996]. Conversely, the incidence of the more severe AIS2 and AIS3 injuries was greater in lateral impacts than frontal impacts [Pattimore et a., 1991], and illustrates their point that the lower extremity can incur serious injuries during side impacts and requires further detailed study.

Detailed examination of the lower extremity injuries in these three studies was limited to nearside impacts because the details of farside impacts were constrained by their small sample sizes. For example, in another study Rouhana and Foster (1985) found that right side versus left side impacts were evenly distributed but Thomas and Frampton (1999) found that nearside impacts occurred twice as often as farside impacts. Additionally, nearside impacts (49%) had a greater association with higher severity injuries (AIS2+ or AIS3+) than farside impacts (10%) [Thomas and Frampton, 1999]. The distribution of the nearside impact injuries showed a predominance of pelvic injuries but the other injured lower extremity regions accounted for approximately one-half or more of those reported [Pattimore et al., 1991, Otte, 1996, Thomas and Frampton, 1999]. More detailed investigation of the injuries suggested that a greater proportion of the pelvic injuries were skeletal while the injuries to the thigh, knee, leg, and foot/ankle had a greater proportion of surface injuries (e.g. cuts and bruises) [Pattimore et al., 1991]. Pattimore et al. (1991) assumed that the lower extremity skeletal injuries were, in general, more severe than the surface injuries. Thomas and Frampton (1999) documented specific skeletal injuries to include simple pelvic ring fracture, femoral shaft fracture, trochanter or femoral neck fracture, and tibial shaft or plateau fracture.

The three previous studies included discussion of the crash environment causing the lower extremity injuries. While crash parameters such as delta V, PDOF, collision partner, contacts and intrusion were discussed in all three of these studies, no one study conducted a comprehensive examination of all of these parameters on their respective data sets. These crash environment parameters and their relationship with lower extremity injury are summarized in the following paragraphs.

Injury severity and lower extremity region were correlated to the delta V. Thomas and Frampton (1999) associated a median delta V of 33 km/h and 47 km/h for the AIS2+ and AIS3+ injuries, respectively. Otte (1996) observed that injuries to the knee and above occurred at higher delta V’s (21 to 50 km/h) than injuries below the knee (10 to 20 km/h).

There was little data presented on the principal direction of force (PDOF). Otte’s (1996) study suggested that the majority of vehicle-to-vehicle (27%) and pole (23%) related collisions were oblique. These collisions were not further specified as to the PDOF or associated lower extremity anatomic region injured.

The collision partners investigated were predominantly vehicle-to-vehicle, poles and unspecified roadside objects. Fifty percent of the collisions were vehicle-to-vehicle and 39% were with unspecified roadside objects including poles [Thomas and Frampton, 1999].

Interior contact was directly related to injury distribution while intrusion correlated to injury severity. While Pattimore et al. (1991) attributed 90% of the pelvis and thigh injuries and 28% of the leg and foot/ankle injuries to a contact with the door, 56% of the leg and foot/ankle injuries resulted from contact with the footwell. Pattimore et al. (1991) also found a median intrusion depth of 10 cm for AIS1 injuries versus a median depth of 23 cm for AIS1+ injuries. Otte (1996) also observed that lower extremity injuries were associated with an intrusion depth of at least 20 cm.

These previous analyses had several limitations. The three studies summarized above were based on the investigation of two data sets only, one in the UK (CCIS) and one in Hannover Germany. Secondly, farside impacts were not further analyzed with respect to delta V, PDOF, collision partner, interior contact and intrusion. Lastly, PDOF and related injury was not fully explored. The specific direction of the impact force was not quantified and was not identified with respect to the type and region of lower extremity injury. These limitations produce a fragmented picture of the crash environment and lower extremity injury relationship, primarily because of the limited PDOF and farside impact data.

The current study will use the NASS/CDS database to investigate the lower extremity injuries sustained by the US driving population and to correlate those injuries with the impact conditions. This will be compared to the previous studies. This study also endeavors to build upon the understanding of injury mechanisms presented in previous studies.

METHODS

The Crashworthiness Data System (CDS) of the National Automotive Sampling System (NASS) database in the US was used to investigate the causes of lower extremity injuries due to side impacts. This study queried the NASS database for the years spanning from 1993 to 2000. The cases were limited to those where the vehicle was a car or light truck and was involved in a tow away crash. However, no restrictions were placed on restraint usage for the injured occupants. This study took an iterative approach to investigate side impact related lower extremity injury. Initially, all injuries were identified to determine comparative yearly proportions between the lower extremity and the other body regions. Next, these data were subdivided into lower extremity injuries for all AIS severities sustained in side or frontal impacts to obtain comparative results for these damage locations, regardless of principal direction of force (PDOF), because the mechanisms of frontal impact injuries to the lower limb have been well documented. Lastly, detailed analyses of the side impact cases were obtained by limiting the specific area of damage to side impacts with an impact direction between 8 to 10 (left side impact) and 2 to 4 o’clock (right side impact). The side impact cases were then reviewed for specifics regarding the crash environment (total delta V, PDOF, intrusion, collision partner, and interior vehicle contacts). All proportions are based on NASS weighted data. Statistical comparison was conducted with tests of two proportions and significance set at a p-value equal to 0.05.

Several terms used throughout the paper require definition for clarity. Five anatomic regions were defined for the lower extremity to aid in identifying the gross location of the injury. These are the pelvis/hip, thigh, knee, leg and foot/ankle. The pelvis/hip region included the bony structures of the pelvis and the femur proximal to, and including, the greater trochanter. The thigh included the supracondylar area of the femur and femoral shaft distal to the greater trochanter. The knee joint included the bony articulations of the femoral condyles, patella and tibial plateau. The leg included the tibial shaft distal to the plateau but proximal to the medial malleous and the fibular head, neck and shaft proximal to the lateral malleous. The foot/ankle included all the bones of the foot as well as the medial and lateral malleoli. It should be noted that the grouping of these lower extremity regions were arbitrarily chosen to identify the gross anatomical regions of the lower extremity. Two additional lower extremity terms were defined for injuries to the femur, tibia or fibula that were not further specified (NFS) by the AIS coding manual, but could have occurred in one of the adjacent regions. These were NFS Knee-Thigh-Hip and NFS Knee-Leg-Foot/Ankle. Frequency is defined as the NASS weighted number of observations of a given injury condition. Proportion is defined as the frequency divided by the total observed injuries in a given impact situation. The regional injury proportion is defined as the frequency divided by the total observed injuries for a specific anatomical region of the lower extremity, which is a subset of the total injury observations in a given impact situation.

RESULTS

The lower extremity is the most frequently injured body region when considering all AIS injuries spanning the years 1993 to 2000. Over the study period, lower extremity injuries accounted for approximately 22% of the total injuries recorded (Figure 1). This is equivalent to approximately 1.3 million injuries to the lower extremity per year in the US. The lower extremity is the 2^nd most injured body region behind facial injuries for AIS1 injuries (Table 1). However, the lower extremity is the leading injured body region for both AIS2 and AIS3 injuries. The lower extremity represents 21.2%, 28.2% and 26.4% of the AIS1, AIS2 and AIS3 injuries, respectively. The AIS1 injuries to the lower extremity are typically abrasions, contusions and lacerations while the AIS2 and AIS3 injuries are mostly bone and joint injuries.

Fig. 1.

Yearly weighted injuries compared by body region for the years 1993 to 2000.

(All car or light truck tow away frontal, side and rear impacts and AIS1 to AIS7 injuries. AIS7 is injured, unknown severity)

Table 1.

Distribution of Body Region Injuries by AIS Severity for the Years 1993–2000

Region	AIS1	AIS2	AIS3	AIS4	AIS5	AIS6	AIS7	Total
Head	3188708	770718	306260	176002	91023	14005	84271	4630986
	6.76	1.63	0.65	0.37	0.19	0.03	0.18	9.82
Face	9218789	300301	44196	851	0	0	1022	9565161
	19.55	0.64	0.09	0.00	0.00	0.00	0.00	20.29
Neck	766836	11714	1068	303	438	398	12814	793573
	1.63	0.02	0.00	0.00	0.00	0.00	0.03	1.68
Thorax	3766742	274522	348700	125857	37772	14937	49858	4618387
	7.99	0.58	0.74	0.27	0.08	0.03	0.11	9.80
Abdomen	1360222	263752	53007	47792	20375	648	50960	1796756
	2.89	0.56	0.11	0.10	0.04	0.00	0.11	3.18
Spine	5250002	311366	68574	8638	10521	3213	5209	5657524
	11.14	0.66	0.15	0.02	0.02	0.01	0.01	12.00
Upper Extremity	8531469	746378	202434	0	0	0	9213	9489494
	18.10	1.58	0.43	0.00	0.00	0.00	0.02	20.13
Lower Extremity	8744877	1049714	369064	1007	485	0	2584	10170000
	18.55	2.23	0.78	0.00	0.00	0.00	0.01	21.57
Total	41250000	3728655	1395020	360867	162424	37797	215932	47150000
	87.48	7.91	2.96	0.77	0.34	0.08	0.46	100.00

(Top value is NASS weighted injury frequency; bottom value is proportion. Bold face indicates the highest percentage for each AIS. AIS7 is injured, unknown severity.)

The lower extremity injuries were also identified by the anatomic structure injured and AIS severity for side and frontal impacts, inclusive of all directions of force (Tables 2 and 3). The surface injuries to the skin/subcutaneous tissue/muscle represent approximately 96% of the AIS1 injuries in both side and frontal impacts. The more severe skeletal (bone and joint) injuries represent 96% to 99% of the AIS2 and AIS3 injuries in side impacts, respectively, and were almost identical for the frontal impacts.

Table 2.

Side Impact Injury of the Lower Extremity Anatomic Structures Compared by AIS Severity for the Years 1993–2000

Anatomic Structure	AIS1	AIS2	AIS3	AIS4	AIS5	AIS7	AIS7
Traumatic Injury	0	48	784	0	0	168	1001
	0.00	0.00	0.05	0.00	0.00	0.01	0.06
Vessels	196	0	208	0	0	0	405
	0.00	0.00	0.01	0.00	0.00	0.00	0.02
Nerves	0	1184	0	0	0	0	1184
	0.00	0.07	0.00	0.00	0.00	0.00	0.07
Soft Tissue	14038	2330	0	0	0	0	16369
	0.85	0.14	0.00	0.00	0.00	0.00	0.99
Skeletal	40808	115673	70357	85	49	0	226973
	2.47	6.99	4.25	0.01	0.00	0.00	13.72
Skin and Subcutaneous	1407820	594	0	0	0	0	1408415
	85.10	0.04	0.00	0.00	0.00	0.00	85.13
Total	1462863	119831	71350	85	49	168	1654347
	88.43	7.24	4.31	0.01	0.00	0.01	100.00

(Top value is NASS weighted frequency for the structure; bottom value is proportion. AIS7 is injured, unknown severity. Number of unweighted injuries is 8330.)

Table 3.

Frontal Impact Injury of the Lower Extremity Anatomic Structures Compared by AIS Severity for the Years 1993–2000

Anatomic Structure	AIS1	AIS2	AIS3	AIS4	AIS5	AIS7	Total
Traumatic Injury	0	544	488	225	0	418	1677
	0.00	0.01	0.01	0.01	0.00	0.01	0.04
Vessels	127	11	276	0	0	0	415
	0.00	0.00	0.01	0.00	0.00	0.00	0.01
Nerves	0	2267	0	0	0	0	2267
	0.00	0.06	0.00	0.00	0.00	0.00	0.06
Soft Tissue	14976	11701	201	0	0	0	26878
	0.40	0.31	0.01	0.00	0.00	0.00	0.71
Skeletal	116056	477018	146180	117	36	0	739408
	3.06	12.60	3.86	0.00	0.00	0.00	19.52
Skin and Subcutaneous	3015174	977	201	0	0	0	3016354
	79.62	0.03	0.01	0.00	0.00	0.00	79.65
Total	3146332	492520	147349	343	36	418	3787000
	83.08	13.01	3.89	0.01	0.00	0.01	100.00

(Top value is NASS weighted frequency for the structure; bottom value is proportion. AIS7 is injured, unknown severity. Number of unweighted injuries is 19619.)

The side impact injuries discussed so far have been for all principal directions of force. However, there are a number of impacts that may not have a large lateral component of force. For this reason, the side impacts were limited to only those impacts with a PDOF between a clock heading of 8 to 10 or 2 to 4. From this point forward, the analyses in this study are restricted to these impact directions. The anatomic structures were again identified by AIS severity (Table 4). The AIS2 and AIS3 bone and joint injuries account for approximately 97% of the injuries. This shows that the more severe injuries are to the bone and joints of the lower extremity. It can be noted that all of the AIS4 and AIS5 injuries were in the 2–4/8–10 o’clock impacts.

Table 4.

Injured Lower Extremity Anatomic Structure by AIS Severity for Side Impact Directions of Force of 2, 3, 4 and 8, 9, 10 o’clock (1993–2000)

Anatomic Structure	AIS1	AIS2	AIS3	AIS4	AIS5	AIS7	Total
Traumatic Injury	0	45	423	0	0	155	624
	0.00	0.00	0.03	0.00	0.00	0.01	0.05
Vessels	128	0	140	0	0	0	268
	0.01	0.00	0.01	0.00	0.00	0.00	0.02
Nerves	0	1122	0	0	0	0	1122
	0.00	0.09	0.00	0.00	0.00	0.00	0.09
Soft Tissue	6099	1067	0	0	0	0	7166
	0.47	0.08	0.00	0.00	0.00	0.00	0.55
Skeletal	34811	95212	62332	85	49	0	192489
	2.69	7.36	4.82	0.01	0.00	0.00	14.88
Skin and Subcutaneous	1091812	547	0	0	0	0	1092360
	84.37	0.04	0.00	0.00	0.00	0.00	84.42
Total	1132850	97993	62894	85	49	155	1294027
	87.54	7.57	4.86	0.01	0.00	0.01	100.00

(Top value is NASS weighted injury frequency for the type of structure; bottom value is proportion. AIS7 is injured, unknown severity. Number of unweighted injuries is 6568.)

In addition, an estimated 808,270 occupants were injured with a total of 1,294,027 injuries to the lower extremity for the study period. The driver (71.9%) was the most frequently injured occupant, followed by the right front passenger (20.5%), the second row right passenger (3.3%) and the second row left passenger (1.7%). This is consistent with the occupancy rates of these seating locations but with a 9% overrepresentation of drivers in terms of injury rate [Rouhana and Foster 1985]. The primary locations of injured occupants were in the outboard seating positions of the first and second rows.

The side impact lower extremity data set was divided into nearside and farside impacts with only the AIS2 and AIS3 skeletal injuries. The AIS4 and AIS5 injuries will not be discussed further because the numbers of these are negligible compared to other AIS4/5 injuries, whereas, the number of skeletal AIS2/3 lower extremity injuries were greater than all other AIS2/3 injuries and are associated with long-term impairment. This subset of the lower extremity injuries had an estimated 47,739 occupants with 150,768 skeletal injuries from nearside and farside impacts during the study period (Table 5). Injuries to the left outboard-seated occupants struck on the left side of the vehicle or right outboard-seated occupants struck on the right side of the vehicle were considered to be from nearside impacts. Conversely, the injuries to the left outboard-seated occupants struck on the right side of the vehicle or right outboard-seated occupants struck on the left side of the vehicle are considered to be from farside impacts. By tests of two proportions, it was found that the nearside first row occupants incurred lower extremity injuries in greater numbers than the farside front row occupants (p<0.05). This was similar for the second row occupants as well. The proportion of nearside drivers, second row left passengers and second row right passengers was higher than their farside counterparts (p<0.05). However, the proportion of nearside right front passengers was not significantly different than farside right front passengers (p>0.05).

Table 5.

Occupants with AIS2 and AIS3 Lower Extremity Injuries from Nearside and Farside Impacts with a PDOF from 2 to 4 and 8 to 10 o’clock (1993–2000)

Seat Position	Impact Location	Occupant Frequency	Occupant (%)	Injury Frequency	Injury (%)
Driver	Nearside	26737	56.01	98450	65.30
	Farside	7013	14.69	20041	13.29
1st Row Right	Nearside	8803	18.44	20593	13.66
	Farside	2586	5.42	6521	4.33
2nd Row Left	Nearside	615	1.29	1279	0.85
	Farside	586	1.23	756	0.50
2nd Row Right	Nearside	1055	2.21	2487	1.65
	Farside	344	0.72	641	0.43

(Number of unweighted occupants is 560 and number of unweighted injuries is 1818)

A detailed analysis of AIS2 and AIS3 lower extremity skeletal injuries was performed. Each injured region and specific structure is shown as a fraction of the total nearside and farside injuries in Table 6. Between the gross anatomic structures, the proportion of pelvis/hip injuries was the highest for both nearside and farside impacts (p<0.05). The proportion of knee injuries was similar to foot/ankle injuries (p>0.05) in both impact modes. The proportion of pelvis/hip injuries decreased from nearside impacts (70.4%) to farside impacts (38.3%) but the opposite observation was made for the thigh (2.8% vs 4.5%), knee (6.2% vs 16.7%), leg (10.1% vs 19.5%) and foot/ankle (5.6% vs 14.7%). All of the above differences in proportions were significant (p<0.05). Within the pelvis/hip region the closed pelvic fractures accounted for 28.1% and 18.1% of the total nearside and farside impact injuries, respectively. The proportion of these pelvic fractures was higher in nearside than farside impacts (p<0.05). The proportion of knee sprains increased from nearside (2.4%) to farside (10.2%) impacts (p<0.05). It is interesting to note that the proportion of fibula shaft fractures decreased from nearside (8.0%) to farside (2.4%) impacts but tibia shaft fractures were negligible in nearside impacts and increased to 15.0% in farside impacts (p<0.05).

Table 6.

Lower Extremity Injuries as a Proportion of the Total Nearside and Farside Injuries for the Gross Anatomic Region and Subdivided into Specific Structures Injured (1993–2000)

Anatomic Region	NS %	FS %	Specific Injured Anatomic Structure	NS (%)*	FS (%)*
Pelvis/Hip	70.4	38.3	Pelvis – fracture NFS	10.6	9.2
			Pelvis – fracture closed	28.1	18.1
			Pelvis – fracture open/displaced/comminuted	21.3	7.6
			Sacroilium – fracture (may include dislocation)	4.9	1.0
			Pubic Symphysis – separation (fracture)	3.5	1.8
Thigh	2.8	4.5	Femur – fracture shaft	2.2	4.0
Knee	6.2	16.7	Knee – laceration into joint	-	2.7
			Knee – sprain	2.4	10.2
			Tibia – fracture condyles	-	1.2
			Tibia – fracture condyles open/displ/comminuted	3.1	1.1
NFS Knee- Thigh-Hip	2.3	2.8	Femur – fracture NSF	2.3	2.8
Leg	10.1	19.5	Fibula – fracture head, neck, shaft	8.0	2.4
			Tibia – fracture shaft	-	15.0
			Tibia – fracture shaft open/displaced/comminuted	1.6	2.0
Foot/Ankle	5.6	14.7	Ankle (Tarsus) Joint – dislocation NFS	-	1.8
			Fibula – fracture lateral malleous	-	2.0
			Fibula – fracture trimalleolar	-	4.3
			Foot – fracture NFS	1.2	4.2
			Metatarsal or Tarsal – fracture	1.1	-
			Tibia – fracture medial malleous	1.7	-
NFS Knee- Leg- Foot/Ankle	2.6	3.4	Fibula – fracture, any type, but NFS as to site	1.8	2.0
Total	100%	100%

^*Proportion of the total AIS2 & 3 injuries from nearside or farside impacts. Specific injured structure proportion from a specific region do not equal region proportions because specific structures with less than 1% of total were omitted. NS = Nearside Impact; FS = Farside Impact. NS weighted/unweighted injury total is 122809/1452 and FS weighted/unweighted injury total is 27959/329.

DELTA V

The majority of the AIS2 and AIS3 skeletal injuries to the anatomic regions of the lower extremity occur at less than 64 km/h (40 mph) total delta V as reported in the NASS database (Figures 2 and 3). The majority of nearside and farside impact pelvic injuries occurred above a delta V of 34 km/h (21 mph). The majority of thigh injuries in nearside impacts were at a delta V greater than 34 km/h (21 mph). The farside thigh injuries occurred at even higher speeds at delta V’s greater than 50 km/h (31mph). Knee injuries and leg injuries occurred in the range from 18 to 32 km/h (11–20 mph) for both the nearside and farside impacts. The delta V for foot/ankle injuries from nearside impacts was also in the range from 18 to 32 km/h (11–20 mph). The majority of farside foot/ankle injuries occurred at delta V’s less than 32 km/h (21 mph).

Fig 2.

Nearside impact lower extremity injuries grouped by delta V (1993–2000).

(All injuries are from the subset of data that includes impacts to the left or right side of the vehicle between 2–4 and 10–8 o’clock with a nearside injured occupant injury severity of AIS2 or AIS3 to the bone or joint of the lower extremity. Nearside weighted/unweighted injury total is 122809/1452)

Fig 3.

Farside impact lower extremity injuries grouped by delta V (1993–2000).

(All injuries are from the subset of data that includes impacts to the left or right side of the vehicle between 2–4 and 10–8 o’clock with a farside injured occupant injury severity of AIS2 or AIS3 to the bone or joint of the lower extremity. Farside weighted/unweighted injury total is 27959/329)

PRINCIPAL DIRECTION OF FORCE

Injury patterns were analyzed for PDOF’s that were 2 to 4 (45º to 135º) and 8 to 10 (225º to 315º) o’clock. The regional injury proportions are presented as the percentage of the total injuries sustained by each anatomic region. The majority of nearside impacts were to the left side of the vehicle with most of the injuries occurring with a rearward component of force (Figure 4). The regional proportion of pelvis/hip (24.3%), knee (47.1%), leg (34.8%) and foot/ankle (25.6%) injuries were highest at a PDOF of 300º. The regional proportion of thigh (24.0%) injuries was highest at a PDOF of 290º. Injuries to the anatomic regions decreased or did not occur as the forward component of the crash force increased. The majority of the farside impacts occurred to the right hand side of the vehicle (Figure 5). Similar to the nearside impacts, the regional proportion of injuries were higher when the PDOF contained a rearward component of force or was perpendicular. However, knee (9.3%) and foot/ankle (10.6%) injuries did occur with a forward component of force at a PDOF of 120º.

Fig 4.

Nearside impact lower extremity injury plotted by PDOF (1993–2000)

(All injuries are from the subset of data that includes impacts to the left or right side of the vehicle between 2–4 and 10–8 o’clock with a nearside injured occupant injury severity of AIS2 or AIS3 to the bone or joint of the lower extremity. Nearside weighted/unweighted injury total is 122809/1452)

Fig 5.

Farside impact lower extremity injury plotted by PDOF (1993–2000)

(All injuries are from the subset of data that includes impacts to the left or right side of the vehicle between 2–4 and 10–8 o’clock with a farside injured occupant injury severity of AIS2 or AIS3 to the bone or joint of the lower extremity. Farside weighted/unweighted injury total is 27959/329)

COLLISION PARTNERS

The most common collision partner in both the nearside and farside crashes is another vehicle (Table 7). This collision partner was the highest in all anatomic regions. The proportion of the vehicle-to-vehicle collision partners decreased from nearside to farside impacts within the pelvis/hip, knee, and NFS knee-leg-foot/ankle regions, but increased within the thigh, leg, foot/ankle and NFS knee-thigh-hip regions (p<0.05). The proportion for the thigh nearly doubled from nearside to farside impacts. The next most frequent collision partner is a tree or pole with a diameter greater than 10 cm and is common in both impact modes. The proportion of thigh injuries due to trees with a diameter greater than 10 cm is atypical of the other regional injury proportions from tree collisions. Trees accounted for 30.6% of the thigh injuries for nearside impacts and decreased to 11.5% for farside impacts (p<0.05).

Table 7.

Proportion of Collision Partners Within Each Lower Extremity Anatomic Region for Nearside and Farside Impacts (1993–2000)

Anatomic Region	Collision Partner	NS (%)*	FS (%)*
Pelvis/Hip	Vehicle	91.4	85.0
	Tree (>10 cm in diameter)	3.1	4.6
	Pole or post (>10 cm but <=30 cm in diameter)	2.3	3.4
	Pole or post (>30 cm in diameter)	-	3.1
	Other traffic barrier (includes guardrail)	-	1.3
	Bridge	1.5	-
	Train	-	1.5
Thigh	Vehicle	36.2	70.4
	Tree (>10 cm in diameter)	30.6	11.5
	Pole or post (>10 cm but <=30 cm in diameter)	6.7	8.4
	Pole or post (>30 cm in diameter)	5.5	9.0
	Other traffic barrier (includes guardrail)	1.0	-
	Bridge	18.7	-
Knee	Vehicle	91.4	85.2
	Tree (>10 cm in diameter)	3.8	14.5
	Concrete traffic barrier	2.9	-
NFS Knee- Thigh-Hip	Vehicle	57.6	87.1
	Tree (>10 cm in diameter)	24.1	10.3
	Pole or post (>10 cm but <=30 cm in diameter)	10.0	1.4
	Pole or post (>30 cm in diameter)	3.6	1.2
	Other traffic barrier (includes guardrail)	3.8	-
Leg	Vehicle	85.0	91.1
	Tree (>10 cm in diameter)	9.6	4.2
	Pole or post (>10 cm but <=30 cm in diameter)	-	1.4
	Pole or post (>30 cm in diameter)	2.8	3.3
	Bridge	1.7	-
Foot/Ankle	Vehicle	60.8	78.2
	Tree (>10 cm in diameter)	9.0	21.8
	Breakaway pole or post (any diameter)	7.6	-
	Pole or post (>10 cm but <=30 cm in diameter)	1.8	-
	Pole or post (>30 cm in diameter)	2.4	-
	Other fixed object	17.5	-
NFS Knee-Leg- Foot/Ankle	Vehicle	86.3	78.8
	Tree (>10 cm in diameter)	3.5	15.7
	Embankment	5.4	-
	Pole or post (>10 cm but <=30 cm in diameter)	1.6	1.2
	Pole or post (>30 cm in diameter)	3.2	-
	Train	-	7.7

^*Proportion of associated collision partner within each anatomic region and do not add to 100% because collision partner proportions less than 1% of total and unknown collision partners were omitted. NS = Nearside Impact; FS = Farside Impact. NS Weighted/unweighted injuries is 122809/1452. FS Weighted/unweighted injuries is 27959/329

INTERIOR CONTACTS

Interior vehicle contacts were also assessed (Table 8). The nearside injuries above the knee to the thigh and pelvis/hip mainly resulted from similar contacts with the side interior surfaces, hardware and armrests as well as the instrument panel for thigh injuries only. Conversely, the pelvis/hip and thigh injuries in farside impacts occurred as a result of contact to the center console, seat/back support, side interior surfaces, and the instrument panel, particularly for the injuries to the thigh. In both nearside and farside impacts, the knee injuries resulted from contact with the knee bolster and instrument panel, but injuries due to console contacts rose in the farside impact scenario. The majority of leg injuries were from contact with the instrument panel in both impact modes. The injuries were skewed in favor of left instrument panel contact during a nearside impact while the center instrument panel contact was associated with farside impact. Contact with the interior surfaces was also a large contributor to the leg injuries in nearside impacts. Foot/ankle injuries were mainly due to contact with the floor and foot controls in both impact modes and contact to the interior surfaces was the second leading source of injury.

Table 8.

Nearside and Farside Impact Interior Contacts Grouped by Lower Extremity Anatomic Region

Anatomic Region	Interior Contact Point	NS*	FS*
Pelvis/Hip	Steering wheel rim, column, transmission lever, or other attach.	1.6	-
	Instrument panel or knee bolster	1.2	11.2
	Left or right interior surface (incl. hardware and armrests)	73.7	21.2
	Seat, back support, belt restraint webbing or buckle	1.2	23.7
	Other occupants	-	5.9
	Floor (toe pan), console, or foot controls	1.9	17.6
	Exterior object	-	1.8
Thigh	Steering wheel rim, column, transmission lever, or other attach.	1.7	1.8
	Instrument panel or knee bolster	18.1	29.7
	Left or right interior surface (incl. hardware and armrests)	64.2	16.6
	Seat, back support, belt restraint webbing or buckle	-	19.0
	Other occupants	-	1.2
	Floor (toe pan), console, or foot controls	4.0	7.4
	Exterior object or ground	2.5	7.0
Knee	Steering wheel rim, column, transmission lever, or other attach.	4.8	1.1
	Instrument panel or knee bolster	70.3	52.8
	Left or right interior surface (incl. hardware and armrests)	5.2	1.2
	Seat, back support, belt restraint webbing or buckle	-	10.2
	Other occupants	-	-
	Floor (toe pan), console, or foot controls	-	34.6
	Exterior object	-	-
NFS Knee- Thigh-Hip	Steering wheel rim, column, transmission lever, or other attach.	3.1	8.8
	Instrument panel or knee bolster	6.7	19.0
	Left or right interior surface (incl. hardware and armrests)	50.4	13.7
	Seat, back support, belt restraint webbing or buckle	6.6	-
	Other occupants	-	-
	Floor (toe pan), console, or foot controls	3.3	11.4
	Exterior object	-	-
Leg	Steering wheel rim, column, transmission lever, or other attach.	-	-
	Instrument panel or knee bolster	42.4	88.2
	Left or right interior surface (incl. hardware and armrests)	38.6	3.7
	Seat, back support, belt restraint webbing or buckle	-	1.4
	Other occupants	-	-
	Floor (toe pan), console, or foot controls	2.5	4.0
	Exterior object	-	1.4
Foot/Ankle	Steering wheel rim, column, transmission lever, or other attach.	-	-
	Instrument panel or knee bolster	2.4	5.8
	Left or right interior surface (incl. hardware and armrests)	38.2	37.5
	Seat, back support, belt restraint webbing or buckle	-	-
	Other occupants	1.2	-
	Floor (toe pan), console, or foot controls	49.6	46.8
	Exterior object	2.0	-
NFS Knee-Leg- Foot/Ankle	Steering wheel rim, column, transmission lever, or other attach.	1.0	-
	Instrument panel or knee bolster	36.8	27.5
	Left or right interior surface (incl. hardware and armrests)	34.3	-
	Seat, back support, belt restraint webbing or buckle	1.3	-
	Other occupants	-	-
	Floor (toe pan), console, or foot controls	1.3	31.7
	Exterior object	-	1.2

^*Proportion of associated interior vehicle contact within each anatomic region and do not add to 100% because proportions less than 1% of total and unknown contacts were omitted. NS = Nearside Impact; FS = Farside Impact. NS weighted/unweighted injuries is 122809/1452. FS Weighted/unweighted injuries is 27959/329

INTRUSION

The dominant crush direction for the side impacts was in the lateral direction but there were also some longitudinal and vertical crush directions reported. The intrusion location varies among the seating positions identified in this study. The trend indicated that a greater number of the intrusions were located at the right outboard seating positions for farside impacts. The intrusion locations were evenly spread between the front left and right outboard seating positions for the nearside impacts. The nearside intrusion depths were predominantly from 15 to 30 cm, but there were reported intrusions of greater than 60 cm. The farside impact intrusion depths were typically greater than the nearside impacts and the majority ranged from 30 to 46 cm deep. In this crash mode, there were also intrusion depths greater than 60 cm noted. The nearside pelvis/hip and thigh injuries were frequently due to left and right side interior surface contacts and accompanied by intrusions of the A/B-pillars and front doors. The intruding components are harder to associate with the contacts for the farside injuries, but the main intruding components were A and B pillars, doors, floor pans and seat cushions in impacts that resulted in pelvis/hip and thigh injuries. The intruding components for nearside knee injuries were mostly the A/B pillars, the front doors, floor pans and seat cushions with an increase in left instrument panel intrusion. The intrusion components were similar for the farside impacts with the exception that there was an increase in instrument panel intrusion. The nearside impact intruding components resulting in lower leg and foot/ankle injuries were similar to those listed above but floor pan and left instrument panel intrusion increased. There was also an increase in toe pan intrusion for the set of foot/ankle injuries. The farside impact intruding components involved the instrument panel intrusion, floor pans, A/B pillars, and the front doors.

DISCUSSION

This study was modeled to investigate lower extremity injuries and crash characteristics with an emphasis placed on investigation of lower extremity injuries arising from nearside and farside impacts relative to the crash characteristics. Farside impacts are included because previous studies were unable to document this crash mode in as much detail as nearside impacts due to sample size concerns. Ultimately, the goal of this study is to develop a better understanding of side impact lower extremity injury to help identify possible injury mechanisms in the crash environment.

The lower extremity is the most frequently injured body region with respect to more serious (AIS2/3) injuries in the US crash data. These AIS2/3 injuries are predominantly skeletal (e.g. bone and joint injuries). However, Pattimore et al. (1991) suggest that in Europe the lower extremity is second to head and face injuries for these same AIS severities but do not identify whether they are skeletal only or a combination of surface and skeletal injuries. Differences in the data may be attributable to the different data collections schemes used.

There is an agreement with the previous studies suggesting that the pelvis is the most frequent injury experienced in side impacts [Pattimore et al. 1991, Otte 1996, Thomas and Frampton 1999]. While there are some differences as to which is the next most frequently injured anatomic region, two of the previous studies suggest that the leg is second to pelvis injuries especially for the more severe skeletal injuries [Pattimore et al. 1991, Otte 1996]. Pelvic injuries from nearside impacts were greater than from farside impacts. With this decrease in the farside impact pelvic injuries, the knee, leg and foot/ankle injuries each increased approximately 10%. The reduction in pelvic injuries from nearside to farside impacts may be because the occupant is located on the opposite side of the vehicle and the intruding side interior surfaces are not impinging directly on the pelvis.

Thomas and Frampton (1999) showed that the nearside impact condition was more common than farside impact. This study suggests that nearside impacts account for 77% of the side impacts. The current study suggests that 72% of the occupants injured in side impacts are drivers. Thomas and Frampton (1999) report that the proportion of injured drivers may be closer to 50%, but this is our estimate elucidated from their study. Rouhana and Foster (1985) noted that the differences found between crash data analyzed on UK and US data sets, for left side and right side impacts, could be explained by the fact that “the driver in Great Britain is a right front occupant.” Another potential reason for the difference noted above is the different restraint usage in the US versus the UK. This study did not discriminate for restraint usage, whereas Thomas and Frampton (1999) did.

Lateral impacts causing injuries to the lower extremity above the knee were associated with larger changes in velocity than the injuries sustained below the knee. Otte (1996) found that the above-the-knee injuries occurred at delta V’s from 21 to >50 km/h and the below-the-knee injuries <10 to 20 km/h. The current study identified a similar trend in the lower extremity injuries. The injury tolerance of the femur and tibia may explain why the thigh injuries occurred at a higher delta V, assuming that the mechanism of injury was lateral bending of these long bones. The lateral static bending strength of the femur is greater than the tibia [Nyquist 1986]. In Nyquist’s (1986) study, the bending moment causing femur fracture was 310 Nm while tibia fracture occurred at 207 Nm. There was also a high proportion of fibula fractures, which is the smallest long bone in the leg and weaker than the tibia. The difference in loading conditions between these long bones and the pelvis/hip make it difficult to postulate why the pelvis/hip injuries also occur at higher delta V’s than the below-the-knee injuries. The ring like geometry of the pelvis may contribute to the structural strength and explain this finding.

While the crash investigations of the previous studies contained impacts with the same directions of force, most did not report on the relationship between the injuries and PDOF. The majority of the injuries to each anatomic region of the lower extremity occurred at a PDOF with a rearward component of force. This suggests that the side impacts were mainly oblique to lateral impacts, which is in agreement with the study by Otte (1996). There were a relatively small number of injuries that occurred at a PDOF with a forward component of force. As suggested by Rouhana and Foster (1985), the presence of the longitudinal component of force suggests that the injury mechanisms may be similar to frontal impacts, especially for the farside impacts where there is no direct contact with an intruding surface to cause localized loading. This may be evidenced by the decrease in the number of pelvis/hip injuries and the subsequent increase of knee, leg and foot/ankle injuries. Overall, while for nearside impacts the greatest source of contact is generally the right and left interior, in farside impacts the greatest source of contact for thigh, leg and NFS knee-thigh-hip was the instrument panel and knee bolster. This confirms Rouhana and Foster’s (1985) finding that occupants in farside impact typically have forward components to their velocity at impact.

Similar to the previous studies, the results from the analysis of collision partners showed that the majority of the side impact injuries were due to vehicle-to-vehicle collisions. Thomas and Frampton (1999) only identified other collision partners as roadside objects that were “narrow” (< 41 cm wide) or “wide” (>41 cm wide). They found that collisions to narrow and wide objects were split almost 50/50. The current study found that the majority of roadside objects were trees and poles with diameters less than 30 cm. Also, this study shows that the pelvis, knee, leg and foot/ankle are mainly due to vehicle-to-vehicle in both nearside and farside impacts. There were a few exceptions. The regional proportion of thigh injuries was similar for the vehicle and tree collision partners in nearside impacts. Also, an elevated proportion of tree and pole collision partners for the knee and foot/ankle injuries was sustained in far-side impacts.

Pattimore et al. (1991) concluded that the pelvis/hip and femur, or above-the-knee, injuries result from the contact with the interior surface or the doors. They also concluded that the below-the-knee injuries arose from contact with the floor pan and to a lesser extent contact with the door. It should be noted, that these injury producing interior vehicle contacts are those from nearside impacts only. This study shows that the interior contact is dependent upon the type of impact, nearside or farside. The pelvis/hip contacts were to the left and right interior surfaces, with and without hardware or armrest involvement, for nearside impacts. On the other hand, the farside impact contacts were from the center console, seat/back support, the interior surfaces and instrument panel. Suggesting that the other interior contacts identified are occurring when the occupant travels toward the center of the vehicle during the impact. Also, the greater intrusion depths for the farside impact condition suggest that the occupant may contact the opposite interior surface during this type of impact. The thigh injuries were associated with similar interior contacts described for the pelvis/hip with the addition of left instrument panel involvement in nearside impact and center instrument panel involvement in farside impact. The knee contacts were to the knee bolster and left instrument panel in the nearside impacts and shifted to the center instrument panel when there was a farside impact. The nearside leg contacts included the side interior surfaces and left instrument panel. The farside leg contacts shifted to the center instrument panel and floor with few reported contacts with the side interior surfaces. The foot/ankle contact was predominantly from the floor or foot controls and side interior surface in either nearside or farside impact. Again, for the thigh, knee, leg, and foot/ankle the contacts appear consistent with the relative motion of the occupant with respect to the vehicle, in that the impact force frequently contains a rearward longitudinal component. It is clear that the current study follows the pattern of nearside contacts identified in nearside collisions for above-the-knee and below-the-knee injuries. This study includes the farside condition and shows the contacts shift towards the center or opposite side of the vehicle as would be expected, but it may not exclude possible rebound injuries for contacts that were not consistent with the assumed motion of the occupant.

In addition, it was previously noted that the more severe skeletal injuries were associated with intrusion [Pattimore et al. 1991]. Otte (1996) concluded that side impacts with at least a 20 cm of intrusion depth were associated with a lower extremity injury. While this may not always be the case, the majority of the nearside impact injuries in the current study were associated with an intrusion depth from 15 to 30 cm. From nearside to farside impacts the range of intrusion depth increased to 31 to 46 cm, perhaps indicating that more energy is required to cause lower extremity injury in farside impacts. In both cases there were some intrusions in excess of 60 cm. The elevated intrusion depths for the farside impacts may be responsible for occupant contacts to the opposite side of the vehicle, as discussed above. The results suggest that the pelvis/hip and thigh injuries were associated with door, A, B or C pillars, floor pan and seat cushion. This may indicate a higher proportion of these injuries are also associated with intrusion into the passenger compartment between the A and C pillars. For injuries that occur in the knee, leg and foot/ankle, there is more involvement with instrument panel and floor pan intrusion. While not investigated, this suggests that the impact location, and subsequent intrusion, may be to the forward half of the vehicle with an oblique PDOF. Below-the-knee injuries may be associated with combined loading from the intruding door and floor pan, which may be a more severe loading condition than direct impact to femur and pelvis. As a result these intrusions may cause injury to the leg and foot/ankle at lower delta V’s. Further, in the foot/ankle region there is a higher proportion of malleoli fractures suggesting that intrusion and occupant kinematics may be producing large ankle moments in both inversion and eversion. The direction of the impact and relative occupant location may be associated with the resultant bending moments produced.

CONCLUSIONS

In summary the conclusions taken from this study are:

1. The lower extremity was the most frequently injured body region for AIS2 and AIS3 injury severities in side impact. These were mainly skeletal injuries involving both the bones and joints.

2. Nearside impacts represented approximately three quarters of the side impact collisions. The pelvis/hip was the leading injury in both the nearside and farside impacts, but the proportion of farside impact pelvis/hip injuries was half that in nearside impacts. This reduction of pelvis/hip injuries was offset by an increase in the proportion of leg, knee and foot/ankle injuries in farside impacts.

3. Farside occupant kinematics appeared to have a forward component of motion causing contact with forward interior components and increased injuries to the knee and below compared with nearside occupants.

4. Farside impacts in this study may have been more severe than nearside impacts. The intrusion depths were generally deeper.

5. Lower extremity injuries above the knee were associated with a higher delta V than those located below the knee. The relative strengths of the long bones of the thigh and leg may explain this finding. In addition, combined loading due to floor pan and instrument panel intrusion may induce a complex loading condition in the leg resulting in injuries that may occur at a lower delta V.

6. The injuries sustained in side impact were typically due to an oblique impact angle with a component of force directed rearward. Some injuries observed were similar to frontal impacts. There is evidence of contact with frontal components and intrusion of the floor pan, which are common in frontal impacts.

7. Above-the-knee injuries were generally associated with intrusion of the door and side structures, suggesting passenger compartment intrusion. Below-the-knee injuries were generally associated with floor pan and instrument panel intrusion, suggesting intrusion to the forward half of the vehicle with an accompanying oblique PDOF.