Intramedullary nails are commonly used to repair femoral fractures. Fractures in normal healthy bone often occur in the young during motor vehicle accidents. Although clinically beneficial, bone refracture and implant failure persist. Large variations in human femur quality and geometry have motivated recent experimental use of synthetic femurs that mimic human tissue and the development of increasingly sophisticated theoretical models. Four synthetic femurs were fitted with a T2 femoral nailing system (Stryker, Mahwah, New Jersey, USA). The femurs were not fractured in order to simulate post-operative perfect union. Six configurations were created: retrograde nail with standard locking (RS), retrograde nail with advanced locking 'off' (RA-off), retrograde nail with advanced locking 'on' (RA-on), antegrade nail with standard locking (AS), antegrade nail with advanced locking 'off' (AA-off), and antegrade nail with advanced locking 'on' (AA-on). Strain gauges were placed on the medial side of femurs. A 580 N axial load was applied, and the stiffness was measured. Strains were recorded and compared with results from a three-dimensional finite element (FE) model. Experimental axial stiffnesses for RA-off (771.3 N/mm) and RA-on (681.7 N/mm) were similar to intact human cadaveric femurs from previous literature (757 + 264 N/mm). Conversely, experimental axial stiffnesses for AS (1168.8N/mm), AA-off (1135.3N/mm), AA-on (1152.1 N/mm), and RS (1294.0 N/mm) were similar to intact synthetic femurs from previous literature (1290 +/- 30 N/mm). There was better agreement between experimental and FE analysis strains for RS (average percentage difference, 11.6 per cent), RA-on (average percentage difference, 11.1 per cent), AA-off (average percentage difference, 13.4 per cent), and AA-on (average percentage difference, 16.0 per cent), than for RA-off (average percentage difference, 33.5 per cent) and AS (average percentage difference, 32.6 per cent). FE analysis was more predictive of strains in the proximal and middle sections of the femur-nail construct than the distal. The results mimicked post-operative clinical stability at low static axial loads once fracture healing begins to occur.

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BackgroundThe bone loss associated with revision surgery or pathology has been the impetus for developing modular revision total hip prostheses. Few studies have assessed these modular implants quantitatively from a mechanical standpoint.MethodsThree-dimensional finite element (FE) models were developed to mimic a hip implant alone (Construct A) and a hip implant-femur configuration (Construct B). Bonded contact was assumed for all interfaces to simulate long-term bony ongrowth and stability. The hip implants modeled were a Modular stem having two interlocking parts (Zimmer Modular Revision Hip System, Zimmer, Warsaw, IN, USA) and a Monoblock stem made from a single piece of material (Stryker Restoration HA Hip System, Stryker, Mahwah, NJ, USA). Axial loads of 700 and 2000 N were applied to Construct A and 2000 N to Construct B models. Stiffness, strain, and stress were computed. Mechanical tests using axial loads were used for Construct A to validate the FE model. Strain gages were placed along the medial and lateral side of the hip implants at 8 locations to measure axial strain distribution.ResultsThere was approximately a 3% average difference between FE and experimental strains for Construct A at all locations for the Modular implant and in the proximal region for the Monoblock implant. FE results for Construct B showed that both implants carried the majority (Modular, 76%; Monoblock, 66%) of the 2000 N load relative to the femur. FE analysis and experiments demonstrated that the Modular implant was 3 to 4.5 times mechanically stiffer than the Monoblock due primarily to geometric differences.ConclusionsThis study provides mechanical characteristics of revision hip implants at sub-clinical axial loads as an initial predictor of potential failure.

The success of platform companies often depends on their ability to “scale” their customer and supplier base. Existing studies have focused on a variety of approaches that platforms may use to scale but have not systematically considered that platforms might acquire other companies as part of this growth strategy. In this paper, we study the acquisition patterns of digital platform companies and contrast these with the acquisition patterns of digital non-platform companies. We find that platform companies acquire earlier (shortly after founding) when compared with non-platform companies, and they often first acquire competing platform companies from the same market niche. As platform companies mature, they begin to acquire non-platform companies from other market niches. This contrasts with how acquisitions are made by non-platform companies, as shown in our analysis.

Research Summary “Freemium” product strategies—where a free basic version of a product is offered alongside a full “premium” paid version—are often used by companies to attempt to increase the size of their user base and benefit from network effects. However, there is limited empirical evidence of how using freemium strategies impacts firm revenues. We empirically investigate how the strengthening of network effects on the Apple App Store influenced product sales of firms using freemium strategies, contrasting the impacts on both market leaders and followers. We find that stronger network effects did not on their own lead to greater revenues for market leaders with respect to followers. However, in settings where freemium strategies were used, network effects greatly amplified the advantage of leaders over followers. Managerial Summary Freemium strategies are increasingly common as a way for firms to increase the number of users and benefit from network effects. However, there has not been much empirical investigation in this regard, particularly in cases where products using freemium strategies face competition. In this article, we investigate how a policy by Apple on the App Store that led to stronger network effects, impacted the performance of market leaders and followers depending on whether they used freemium or not. We find that it led to an increase in revenues of leaders over followers (effectively, market dominance) but only when freemium strategies were used. The analysis demonstrates an interplay the between business model and network effects. In addition, it shows how stronger network effects and freemium strategies only benefitted market leaders in our setting.