HIFLEX KNEE PROSTHESIS – DOES HIGH FLEXION TRANSLATES INTO IMPROVED FUNCTION
Total joint replacement is the most technologically advanced solution for arthritic pain, however a search for a better functional and durable prosthesis still continues. The original Total Condylar design was very successful in terms of pain relief and durability but the average post op flexion achieved was only around 900 to 950 [1-7]. Even though this may be enough for most of the daily activities in the western world , Asians and particularly Indians require higher flexion for most of their daily social habits and customs . In the recent times a number of additional design modifications have been introduced to achieve this goal [10, 11]. However how much impact this increase in the flexion has on patient satisfaction is yet to be determined.
Minoda et al  analysed range of motion of standard and hi-flex cruciate retaining prosthesis prospectively . They had 89 knees with standard and 87 knees with high flexion CR total knee prostheses [both Next Gen brands]. Differences in age, gender, diagnosis, preoperative ROM of the knee, and Knee Society Score between the 2 groups were not statistically significant. At 12-month follow-up, average ROM was 112.0° ± 12.6° for standard, and 115.3° ± 13.4° for high-flexion CR prosthesis (P = .101). They found no significant differences between groups with regard to ROM, clinical, or radiographic parameters.
Seon et al  analysed 100 knees with 50 knees in each category of Hi-flex and standard total knee prosthesis . At the time of the final follow-up, the average maximal non-weight-bearing flexion was 135.3⁰ for the knees in the high-flexion group and 134.3⁰ for the knees in the standard group; the difference was not significant. Moreover, no significant difference was found between the groups in terms of weight-bearing flexion (124.8⁰ in the high-flexion group and 123.7⁰ in the standard group) and the number of knees that allowed kneeling and sitting cross-legged. The average Hospital for Special Surgery knee score was 94.4 points in the high-flexion group and 92.4 points in the standard group; the difference was not significant. The Western Ontario and McMaster Universities Osteoarthritis Index scores also showed no significant difference between the groups. Thus no functional difference was noted in two groups.
Nutton et al  performed prospective randomised comparison of the functional outcome in patients receiving either a NexGen LPS-Flex or the standard design . The study included total of 56 patients, half of whom received Hi-flex and standard knee prosthesis each. They found that there was no significant difference in outcome, including the maximum knee flexion, between patients receiving the standard and high flexion designs of this implant.
Gupta et al  reported a significant improvement in the post-operative range of movement using a high flexion rotating platform design when compared with a standard design of rotating-platform TKR . Similarly, Bin and Nam  found a significant improvement in knee flexion at one year after operation in patients receiving a high flexion design compared with a standard knee replacement, particularly in patients with a pre-operative range of flexion of less than 90° .
Kim, Sohn and Kim  were unable to show a significant improvement in knee flexion using a NexGen LPS-Flex knee replacement . In their study, the standard design was used in one knee and high flexion prosthesis in the other. After a mean of 2.1 years the mean range of movement was 136° in the standard design and 139° in the high flexion design, compared with a mean preoperative range of movement of 126° and 127°, respectively. In their Asian population, the pre-operative range of movement was greater than in the present series, despite which they were unable to demonstrate any advantage in using a high flexion design over the standard version. Other studies from Asian centers have failed to show an improvement in knee flexion using a high flexion design [18, 19]. This is in contrast with expectations that the Asian population will be more satisfied with the Hi-flex designs.
Menegheni et al retrospectively reviewed 511 TKAs in 370 patients fitted with posterior cruciate ligament–substituting prosthesis (NexGen Legacy, Zimmer, Warsaw, Ind) of a traditional design (not designed for high flexion) . The mean follow-up was 3.7 years (range, 2-8 years). Regression analysis determined the effect of obtaining high flexion (>125°) on Knee Society, stair, function, and pain scores. Of 511 TKAs, 340 (66.5%) obtained range of motion greater than 115°, and 63 (12.3%) TKAs obtained high flexion greater than 125°. There was no difference between the patients who obtained flexion greater than 115° and those who obtained high flexion greater than 125° in Knee Society scores (P = .34) and function scores (P = .57). Patients with greater than 125° of flexion are 1.56 times more likely to demonstrate optimal stair function (P = .02). Obtaining flexion greater than 125° after TKA does not offer a benefit in overall knee function. However, obtaining a high degree of flexion appears to optimize stair climbing.
First metaanalysis done by Gandhi et al was published in 2009 January . They studied 6 studies that met with their inclusion criteria. They concluded that High-flexion implant design improves overall ROM as compared to traditional implants but offers no clinical advantage over traditional implant designs in primary knee arthroplasty.
Murphy et al  performed a systematic review of published trials designed to determine if there is a significant increase in ROM or function in patients who receive a high-flexion TKA compared to those who receive a standard TKA . Nine studies fitting the inclusion criteria were analysed. They concluded that there was insufficient evidence of improved range of motion or functional performance after high-flexion knee arthroplasty.
The literature produces a very conflicting picture with most of the independent studies concluding that the Hiflex design features do not translate into improved function. How a randomized study in population such as Indian population, for whom squatting and cross legged sitting is quite important, will be more indicative
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